elm_interface_scrollable: freeze the manager object when content moves
this freezes the manager object of the user of the interface. This
improves the overall performance when moving arround focus in a
scrollable object, since no new relations are needed to be recalculated.
This safes up about 2% in the "Scroller 2" test. Additional
optimizations are added in another commit.
Differential Revision: https://phab.enlightenment.org/D7349