[entry] implement undo support
[entry] implement undo support

Implement simple undo support by saving the current text on a
timeout after change and after each space press.

Fixes http://bugzilla.moblin.org/show_bug.cgi?id=4500

clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx