make clipboard open minimal time as wxPython documentation recommends

https://wxpython.org/Phoenix/docs/html/wx.Clipboard.html#wx.Clipboard.GetData

"Call wx.Clipboard.Open to get ownership of the clipboard. If this
operation returns True, you now own the clipboard. Call
wx.Clipboard.SetData to put data on the clipboard, or
wx.Clipboard.GetData to retrieve data from the clipboard. Call
wx.Clipboard.Close to close the clipboard and relinquish ownership.
You should keep the clipboard open only momentarily."

Maybe it makes situation with pretty annoying error 'clipboard already
open' a little bit better.

traceback:
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 955, in OnSaveProjectMenu
self.RefreshAfterSave()
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 946, in RefreshAfterSave
self._Refresh(TITLE, FILEMENU, EDITMENU, PAGETITLES)
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 926, in _Refresh
self.RefreshFunctions[element]()
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 766, in RefreshEditMenu
IDEFrame.RefreshEditMenu(self)
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 1185, in RefreshEditMenu
if self.GetCopyBuffer() is not None:
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 956, in GetCopyBuffer
if wx.TheClipboard.Open():
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_misc.py", line 5793, in Open
return _misc_.Clipboard_Open(*args, **kwargs)
<class 'wx._core.PyAssertionError'>: C++ assertion "!m_open" failed at ../src/gtk/clipbrd.cpp(598) in Open(): clipboard already open

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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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# This file is part of Beremiz, a Integrated Development Environment for

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# programming IEC 61131-3 automates supporting plcopen standard and CanFestival.

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#

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# Copyright (C) 2007: Edouard TISSERANT and Laurent BESSARD

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#

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# See COPYING file for copyrights details.

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#

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# This program is free software; you can redistribute it and/or

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# modify it under the terms of the GNU General Public License

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# as published by the Free Software Foundation; either version 2

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# of the License, or (at your option) any later version.

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#

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# This program is distributed in the hope that it will be useful,

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# but WITHOUT ANY WARRANTY; without even the implied warranty of

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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

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# GNU General Public License for more details.

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#

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# You should have received a copy of the GNU General Public License

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# along with this program; if not, write to the Free Software

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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.

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libraries = [

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    ('Native', 'NativeLib.NativeLibrary'),

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    ('Python', 'py_ext.PythonLibrary'),

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    ('SVGUI', 'svgui.SVGUILibrary')]

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catalog = [

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    ('canfestival', _('CANopen support'), _('Map located variables over CANopen'), 'canfestival.canfestival.RootClass'),

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    ('modbus', _('Modbus support'), _('Map located variables over Modbus'), 'modbus.modbus.RootClass'),

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    ('c_ext', _('C extension'), _('Add C code accessing located variables synchronously'), 'c_ext.CFile'),

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    ('py_ext', _('Python file'), _('Add Python code executed asynchronously'), 'py_ext.PythonFile'),

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    ('wxglade_hmi', _('WxGlade GUI'), _('Add a simple WxGlade based GUI.'), 'wxglade_hmi.WxGladeHMI'),

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    ('svgui', _('SVGUI'), _('Experimental web based HMI'), 'svgui.SVGUI')]

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file_editors = []