Activity data changes: "Revert changes" button in activity panel

[tmillross]

An option for tracking data changes, as related to #10. This suggested funcitonality is for recovering from accidental data changes, mistakes, or the user changing their mind regarding an activity value.

1. On setting activity read-only=false
2. A button “Revert changes” (or similar) becomes visible on activity panel, initially grey(inactive) until a change is made
3. An additional activity object is created in-memory, storing the initial values for the activity being edited
4. When a user clicks the “Revert changes” button (and a dialog to confirm decision): the values from the unedited in-memory activity are used to overwrite the values on the interface which the user has edited
5. The values visible in the activity interface are always still synced to file, and behave as described in other sections.