ve.ui.Toolbar: Emit position event on toolbar instead of surface

mw.ViewPageTarget is currently getting events from both the
platform target toolbar and context menu toolbar because the
event is emitted from within the toolbar to the surface.

Instead we're now emitting it on the toolbar itself and it is up
to the binder to access the correct one and listen to its events.

Bug: 52317
Change-Id: Ibd8053768e82b1df91081bd77a172628ea855db7
Merge "ve.ui.Toolbar: Emit position event on toolbar instead of surface"