Simplify VE inspector code by extending new MWLiveExtensionInspector

All live preview logic has been moved to ve.ui.MWLiveExtensionInspector so
so this can be simplified down to just static configuration.

Depends on I708c4cb012be in VE-MW.

Change-Id: I24b4788e97411eec18161709ca5d47c87d3f64da
(cherry picked from commit c12523f908f16f393eaf7cada0818e37003b881f)