Implemented more of ActionStack interface for ActionStackModel.

* Implemented createRedoAction and createUndoAction.
* Wrapped various signals and slots
Replace manually wired undo/redo actions with fully dynamic ones from the ActionStackModel.

* Decapitalized Action descriptions so they can be used as part of sentences
  and recapitalize them in the model (see comments for rationale)
* QActions generated by the model are now prepended to edit menu and no longer
  hard-coded in the ui file
* Undo/Redo menu actions now have proper undo/redo shortcuts attached to them
* Removed F5/F6 association and corresponding info text