- Modified UBvgSubsetAdaptor to correctly save and load strokes, so the
transform matrices that were saved are now loaded correctly.

- Added handling of mousePress / Move / Release events to
UBGraphicsStrokesGroup, so that the transform matrix is calculated and
stored after moving a pen stroke directly (by clicking on it, and not on
its frame). Note: this duplicates quite a bit of code that is in
UBGraphicsDelegateFrame. It may be best to go back and modify both
classes so that the same functions can be called when moving a stroke.

also corrected the "useSystemOnScreenKeybard" to
"useSystemOnScreenKeyboard".

Mostly, removal of old code that had been commented out instead of
removed

- Moved all platform-specific code to UBPlatformUtils
- For now, removed support (on Linux) for checking whether onboard is 
running when OpenBoard is started (it wasn't working anyway, but it needs a
cleaner solution for all 3 platforms)

Post-install script on Linux: delete OpenBoardUser.config file (avoids a crash if the config file comes from a version <= 1.10a3)

Presumably due to the change in how pen strokes were saved to file (commit
8ed2e245), pen strokes with pressure levels were badly saved. They were
converted to lines instead of polygons, meaning that every time a page
was saved or duplicated, the lines would look worse and worse -- and
artefacts would appear.

This should now be fixed.

(due to commit 9adc8991).

Fullscreening should now work as expected in OSX 10.9 and 10.10, with
one or multiple monitors.

- Linked necessary Windows SDK libraries
- Solved compilation errors upon including `wmsdk.h` or other Windows
SDK headers (`interface` keyword wasn't recognized)