Undo/Redo Feature

[hkirat]

Branch includes work done on the undo/redo feature.
Implementation :

1. GTE.UNDOQUEUE : Global queue that can be accessed that contains Changes Object. On pressing the undo key, the UNDOQUEUE is popped and the object returned is used to undo the set of changes done on the previous click.
2. Changes class : Changes class contains a queue itself of Change objects. It also contains various other attributes that represent the kind of set of changes taking place.
3. Change class : Change class is a single unit of change taking place. Each undoable click consists of a number of events taking place and the change class represent each of these small changes.