So far revisions for AbstractEditorSupport were not needed because until
recently we only had a single snapshot ("global snapshot"). Now, since
editor snapshots are introduced, we need to make sure to update the
corresponding documents in these snapshots, too. To do this efficiently,
a revision is introduced for AbstractEditorSupport.

Task-number: QTCREATORBUG-10894

Change-Id: Ibad1dbbafb7c721d1328959c1e903345fe465326
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>