Rewrite GdkWin32Keymap (V2)
This MR gets rid of the old, slow keymap initalization code. This cuts 600 ms startup latency on my machine. It makes the start of GTK3 applications feel much snappier. (See #2055)
The new code loads the tables directly from the keyboard layout DLLs. This is not only faster, but also allows us to look up keys without clobbering the global keyboard state.