QmlDesigner: refactoring PropertyEditor

* Renaming PropertyEditor to PropertyEditorView
* Adding PropertyEditorWidget
* Renaming DeclarativeWidgetView to QuickPropertyEditorView
* Renaming NodeType to PropertyEditorQmlBackend
* Moving PropertyEditorQmlBackend into separate file
* Cleaning up interface of PropertyEditorQmlBackend

Change-Id: I1291feb5880b01d1e2bac09f4eb1e0181ff04d5c
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: signal was renamed

Change-Id: Iddb2066372cdb9f1b80b8ab53ab88c69273634c1
Reviewed-by: Marco Bubke <marco.bubke@digia.com>