MetaCursorTracker: add support for loading cursors from the theme

Not only this way we get the right Adwaita cursor as the default
(instead of shipping our own in png format), but we also add
support for all MetaCursors as root cursor (which most important
should allow us to have I-beams in shell entries)

https://bugzilla.gnome.org/show_bug.cgi?id=707573