QmlDesigner: Rename AbstractDesignerAction to ActionInterface

We are already in the QmlDesigner namespace and the class is
a pure abstract interface.

Change-Id: Ibabea5f8091fda22a3086ae032af794bc12ebf38
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: Rename DefaultDesignerAction to AbstractAction

We are already in the QmlDesigner namespace and default gives
no real information about the class.

Change-Id: Ia45c93c778d207c5237d52f5bd2bef6ee6559bb5
Reviewed-by: Marco Bubke <marco.bubke@digia.com>