Factor out event handler binding in ve.init.Target

Subclasses that activate and deactivate will need
to unbind and rebind these.

In the future we should really consider putting the
concept of activation and deactivation up in
ve.init.Target rather than down in ve.init.mw.ViewPageTarget.

Change-Id: I4efbae614ee8e9610dbf71f2c238280f5351e45b
Merge "Factor out event handler binding in ve.init.Target"