ui: switch to single file mode with snippet maximize

* switch back to snippet view with snippet button in file title bar
* use new title bar widget in single view as well
* restore view mode on undo, store current view mode for every message id (added id to message)
* treat view mode switch as adapt message on undo stack
* fixed crash after snippet expansion
* fixed hatching in file title not shown

src/lib/component/controller/ActivationController.cpp

@@ -61,6 +61,7 @@ void ActivationController::handleMessage(MessageActivateFile* message)
 		MessageChangeFileView msg(
 			message->filePath,
 			MessageChangeFileView::FILE_MAXIMIZED,
+			MessageChangeFileView::VIEW_CURRENT,
 			true,
 			true
 		);