When changePart() is called, we have two type of actions:

- the ones we want to execute only in case the part was changed by the
  user (and not programatically -- e.g. because the request came from
  the widget)
- the ones we want to execute all the time

Resetting the view (so the tile buffer drops all the tiles) is something
we want to do all the time, that's one of the needed things when the
set-part event is emitted.

Change-Id: Ic6bfbe460dc5a3fd8834f3b1ef93436fc9d3ad84