fix issue, then it wasn't possible to view FBD programs

the reason for that is possible wx-3.0-gtk2 bug that happens if
ALWAYS_SHOW_SB is set.

Traceback (most recent call last):
File "Beremiz.py", line 1045, in OnProjectTreeItemActivated
IDEFrame.OnProjectTreeItemActivated(self, event)
File "IDEFrame.py", line 1667, in OnProjectTreeItemActivated
self.EditProjectElement(item_infos["type"], item_infos["tagname"])
File "IDEFrame.py", line 1752, in EditProjectElement
new_window = Viewer(self.TabsOpened, tagname, self, self.Controler)
File "editors/Viewer.py", line 611, in __init__
EditorPanel.__init__(self, parent, tagname, window, controler, debug)
File "editors/EditorPanel.py", line 68, in __init__
self._init_ctrls(parent)
File "editors/EditorPanel.py", line 52, in _init_ctrls
self._init_Editor(self)
File "editors/Viewer.py", line 603, in _init_Editor
style=wx.HSCROLL | wx.VSCROLL | wx.ALWAYS_SHOW_SB)
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk2/wx/_windows.py", line 296, in __init__
_windows_.ScrolledWindow_swiginit(self,_windows_.new_ScrolledWindow(*args, **kwargs))
PyAssertionError: C++ assertion "scrolled" failed at ../src/gtk/scrolwin.cpp(205) in DoShowScrollbars(): window must be created

fix issue, then it wasn't possible to remove functional blocks from
project using right click and popup menu.

The problem is that, PopupMenu was called using IDEFrame object and as
a result item in ProjectTree loosed focus and focus went to IDEFrame.
Therefore later functions couldn't detect which item in ProjectTree
was selected.

fix many compilation warnings about static variables used by non-static functions

[CC] plc_main.c -> plc_main.o
/tmp/be/build/plc_main.c:396:40: warning: ?LogBuff? is static but used in inline function ?copy_from_log? which is not static
memcpy((char*)buf + remaining, LogBuff[level], size - remaining);
^
/tmp/be/build/plc_main.c:395:22: warning: ?LogBuff? is static but used in inline function ?copy_from_log? which is not static
memcpy(buf, &LogBuff[level][buffpos], remaining);
^
...

^
/tmp/be/build/plc_debugger.c:825:20: warning: ?retain_offset? is static but used in inline function ?BufferIterator? which is not static
Retain(retain_offset, size, real_value_p);
^
/tmp/be/build/plc_debugger.c:823:47: warning: ?retain_offset? is static but used in inline function ?BufferIterator? which is not static
unsigned int next_retain_offset = retain_offset + size;
^
^

make attribute CFLAGS and LDFLAGS optional and add default empty value

Actually CFLAGS and LDFLAGS are not required and can be empty.
Without default empty value if target platform in project settings was
set to "Linux" CFLAGS and LDFLAGS was initialized with NoneType.
The result was broken build and it wasn't to save/load project with
such settings.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 956, in _Build
if not builder.build() :
File "/home/developer/WorkData/PLC/beremiz/beremiz/targets/toolchain_gcc.py", line 96, in build
Builder_CFLAGS = ' '.join(self.getBuilderCFLAGS())
TypeError: sequence item 0: expected string, NoneType found