Undo redo memento

-Capped the undo/redo stack to 10 entries
-Prevented undo/redo from throwing an error if the stacks are empty
-Fixed bug which was causing a redo to draw the last moved class red

Current undo/redo status:
Working: new class, new relationship, move class, move relationship, rename class, copy/paste, add member, add method
Not yet working: new diagram, move class w/ attached relationship