GDK W32: Cache multiple keyboard layouts simultaneously

GDK W32: Cache multiple keyboard layouts simultaneously

This changes the group/level semantic.
Previously W32 backend used "group 0/1" to denote "AltGr OFF/ON"
and "level 0/1" to denote "Shift is OFF/ON".
Now "group" means "keyboard layout" and there can be up to 255 groups,
while AltGr and Shift are combined into a single level enum that
takes values between 0 and 4.
Unlike X, W32 doesn't do effective group overriding, meaning that
it will never tell the caller that a different group was actually
used (even for universal keys, such as Enter), because key symbol
table is completely fabricated and there's no point in trying to
save a few of kilobytes of RAM by not duplicating universal key
records for all groups.

Also contains many whitespace changes (tab elimination, fixed
indentation) and cleanup (axed a few global variables, these are
now accessed via the default keymap).

https://bugzilla.gnome.org/show_bug.cgi?id=768722