Add `Undo` and `Redo` controls to an `Edit` dropdown menu in the menu bar.

[Peregrine05]

Chunky should have Undo and Redo controls located in an Edit dropdown menu, and they should be able to undo or redo any undoable or redoable action that the user performs. The current Undo and Redo controls apply only to input fields, and they become inaccessible once an input field loses focus.

Related to #1387