Multiple changes to reduce the amount of unprefixed symbols

Multiple changes to reduce the amount of unprefixed symbols

The X11 backend exports a number of symbols which are _-prefixed
(so don't become part of the gdk api), but are not named in a
way to prevent accidental clashes between backends.

The one API change here is that the gdk_xid_table functions
have been removed - they did not server an purpose, since the
xid table only stores windows anyway, and we already have a
lookup-by-xid function for windows.