Clean up the source tree

Remove a lot of old, unmaintained files
Move Wayland protocols into src/wayland
display: Fix a typo
Remove meta_core_increment_event_serial

It's unused.
window: Move type_atom to window-x11
window: Make window->desc a bit clearer

And remove the hack for the "broken GNU libc". It works fine on
my system here.
Move MetaGroup to be X11-specific