C++: release more futures.

The CPPEditorWidget retained two futures, as did the attached future
watchers retained them too. Together, each future and the watcher held
on to a complete snapshot that would only get released when another
future was set. This could result into retaining old snapshots in
editors that were invisible/unused for long.

Change-Id: I1133e857c620437b4a69b9dad468f6bd458304b8
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>