text: Add ::cursor-changed signal

Since we are trying to eliminate the ClutterGeometry type, we should
replace the only entry point still using it: the ::cursor-event signal
of ClutterText.

Instead of passing the cursor geometry, we should add an accessor
function.

The combination of signal and getter for the cursor geometry means that
we can deprecate ClutterText::cursor-event, and mark it for removal in
Clutter 2.0.

https://bugzilla.gnome.org/show_bug.cgi?id=682789