Minimum required Windows version is XP again (instead of Win10).
Win_GetWindowScalingFactor() tries to use two dynamically loaded functions
from newer windows versions (8.1+, Win10 1607+) and has a fallback for
older versions that also seems to work (at least if all displays have
the same DPI).
Moved the function to win_main.cpp so the dynamically loaded functions
can be loaded at startup; so edit_gui_common.cpp could be removed again.
rvGEWindowWrapper is still TODO - it's not as straightforward, because
it insists on storing a pointer in an idWinVar (using idWinInt), but there
is no idWinVar type for pointers
Editor also seems to start, didn't test much further.
Only tested 32bit Windows, I fear the editor code isn't 64bit clean..
I hope I haven't broken anything elsewhere..
