Track "dirty state" and respond accordingly

[jandrieu]

When editing, keep track of whether or not there have been any edits since last save. This is typically marked as a "dirty" status.

If not "dirty", then disable the save option in the menu.
If "dirty", enable the save and also when people quit while dirty, pop-up an alert to save before quitting.

This is NOT MVP, but it's definitely expected in modern editors and, frankly, shouldn't be too hard to support.

[Deleted user]

This issue was moved by jandrieu to satyrnjs/satyrn#24.