Buttons for undoing and redoing

Just made a release. Undo & Redo buttons have been added to the UI. Ctrl/Cmd+Z and Ctrl/Cmd+Z+Shift (or Ctrl/Cmd+y) have always worked to undo and redo, but users were not discovering this naturally despite my initial assumption that they would.

Having buttons in the UI is meant to help ensure that this feature is discovered, and is a part of work I’m doing right now to integrate onboarding/tutorial content into Shmeppy.