Improved the undo/redo system
+ It is now visible which TextLayer the user is editting (if he is editting one)
+ Shortcut keys added for the undo/redo actions (Ctrl+Z and Ctrl+Y)
+ Tools can now indicate whether they're currently editting a layer
* The undo/redo buttons are now enabled/disabled according to whether there are actions that can be undone/redone