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)

Page DPI is now saved as the DPI that was read when opening the file.
While not a perfectly fool-proof fix, it will at least allow files to be
migrated between OB 1.02 and 1.10.