Fix black background in preview window

Problem found and fixed on GNU/Linux
wxWidgets 3.0 built with GTK3+ support and running on Xorg.

Avoid division by zero in case width or height for page are not specified

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3755, in OnPaint
self.DoDrawing(dc)
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3723, in DoDrawing
for y in xrange(self.PageSize[1] - (ystart * SCROLLBAR_UNIT) % self.PageSize[1], int(window_size[1] / self.ViewScale[1]), self.PageSize[1]):
<type 'exceptions.ZeroDivisionError'>: integer division or modulo by zero

IDManager: added import/export plus little cosmetic enhancements.

IDManager : refactored a bit, moved some code into PSKManagement.py. Now captures URI and PSK on new PYRO(S) and propose them when editing URI. Import/export still to be implemented.

Renaming servicetype into scheme.

Added toolbar button to launch ID Manager dialog.

Renamed IDManager control into IDBrowser, because dialog will be named IDManager

IDManager.py : finished selector mode.

Added early implementation of IDManager.py. For now only used to select ID in URIEditor

Path may contain spaces... sequel of d1359cb

'Merged' URI editor dialog and Discovery dialog, also clean up related code

Cut more useless code from URI Editor

URI Editor Dialog : moved it to /dialog, and fixed naming.

URI editor : Rewrote most of it, cleaner.