Add DiffEditorManager, refactoring.

GitDiffSwitcher operates now on IDocument.
GitDiffHandler operates now on DiffEditorController.
"source" property now attached to editor's document (VcsBasePlugin).

Change-Id: Ie2570a597b8b992ac1dc33b9179eca459c8a751a
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>