efl_ui_pan: emit position changed when content is resized

efl_ui_pan: emit position changed when content is resized

even if the position is not really changed here, the min / max relation
has changed. If we do not emit this event here, every user (that
calculates a relative position) would have to monitor the pan position
and the size of the content. This simplifies the given usecase, and
fixes the scroller position when new items are added to the collection.

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D9411