x11: Redo cursor handling completely

x11: Redo cursor handling completely

Instead of creating a GdkX11Cursor, create GdkCursors. Cache the XCursor
in a hash table instead.

Also, make use of the new fallback mechanism for fallback code: Make
sure to provide cursors for the names that are guaranteed to exist, but
do not do bad attempts at displaying texture surfaces.
Black/White/transparent is not a replacement for those.