genlist: Fix rare jump in prepend insert

genlist: Fix rare jump in prepend insert

If an item is selected, and another item is insert before
the selected item, then we try to lock the genlist view (pan)
around the selected item (if it's visible). Unfortunately,
every 16 inserts cause a jump by one line in the scroller.
That's because the scroll math assumes the block position is
known, but since it's a new block it wasn't known.

This patch fixes this issue by precalculating the block position.

Test scenario:
 elementary_test -to "Genlist Tree, Insert Relative"

Select an item, clikck 50 times on "+ before".
The view should not jump.

This does not fix fileselector's craziness.

@fix