Replaced all direct construction of bitmaps and icons using wxBitmap("name") with load_resource_* calls.

New header util/platform.hpp for platform specific stuff that doesn't go elsewhere;
Changed resource names from "CURSOR_SOMETHING" to "cursor/something", so it matches the filenames (in progress for other resource types).

Compiled Linux version. The welcome screen doesn't display the icon images, and the buttons don't work. Data is found at *prefix*/Magic Set Editor/data and the images at *prefix*/Magic Set Editor/resource. On Ubuntu, *prefix* is /usr/local/share. Resource files are all the image files from resource. I haven't configured them, because I haven't made it past the welcome screen.

What is left is mostly:
 - warning: converting double to int
     -> add a cast/to_int or ignore
 - wrong initialization order in ctor
     -> just swap the order to match the class
 - errors about wxCursors
     -> add a function loadResourceCursor