backend: Store XcursorImages for theme cursors

There's a chance the icon will be animated, so store the XcursorImages
instead of the individual XcursorImage, and handle that as a nimages=1
special case.

API to "tick" a cursor animation, and retrieve current frame timing
information has been added.

https://bugzilla.gnome.org/show_bug.cgi?id=752342