Issue #4: Changed operation selection to checkboxes

Changed operation selection to checkboxes and added an "Execute" button
to take care of issue #4. Also added "Select All" and "Select None"
buttons.

Also:
- Switched a large number of user messages to resources.
- Added a basic priority property to the operations to keep them in
order when running multiple
- Added an interface to indicate that a class/control can generate a
sync operation