Fix the saving and loading of the current changeset configuration in the Synchronize view.

There are circumstances when distinct changesets available in the Synchronize view at Eclipse shutdown time are garbled in the next session, meaning that the comments of some of the changesets are lost in the saving and reloading.

I have already identified the cause as being the fact that those changesets share the same name but the code makes the assumption that changeset names saved to file are different. When assigning comments back to each changeset, the first comment found is assigned to all changesets (because they have the same name).

The fix is to add an identification element that is not user-editable so that changesets with the same name (from the user's perspective) will still be perceived as different when saving and reloading the configuration.

This is actually a massive undertaking because the Eclipse preferences API used has been deprecated for a long time. The change in API will also allow removing obsolete entries as part of the post-commit tasks, which is now possible only manually.

The fix to this bug will potentially imply changing the behavior of #482 (closed) where the changeset could be created before the dialog is offered to the user and deleted if the user clicks Cancel.