Undo redo limits

Added a limit of 10 undo/redos
Prevented undo/redo from being called if the stacks are empty
Fixed undo redo red color bug

Fixed bug where performing a redo causes the last moved class to be
drawn as red
Undo redo additions

Undo/redo now working for new diagram
Fixed a undo-action-undo bug
Merge pull request #27 from Zedzap/undo_redo_memento

Undo redo memento