The crash fix simply avoids using a negative index. The label change just fleshes out the use of ButtonType on the UndoButton (used for both un/redo).
Merge remote-tracking branch 'remotes/billlutton/fixUndoRedo'
Squashed commit of James' work - context menu items:

commit d413a5afbcc34276d916e6b03df881b3d2086beb
Author: James Bolding <bolwire@cableone.net>
Date:   Mon Nov 25 23:59:21 2013 -0600

    Added two options to context menu,
    First aligns the effects start and end times with one click.
    Second matches the duration of all selected elements without change to start times.