VCS: Use documents instead of editors a bit more.

This patch mostly gets rid of EditorManager::openedEditors usage. The
VCS editors should have a better widget<>document separation, also to
make it possible to split/duplicate them, but that's for another time.

Change-Id: Idd92a6a4884ff69fba4f4793d182aa7ff68d79e4
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>