Reordering in grid view should check if the cell is "completely" visible

* Have a call in grid view
* Have a participant that is at the bottom of the screen, so the cell is only "partly" visible.
* That participants starts speaking

Expected: Should reorder to the top so it's completly visible
Actual: Nothing happens, as the cell is visible

Start at the grid-view participant reordering path and inspect how it decides whether a speaking participant's cell is visible. Reproduce the case with a partly visible participant at the bottom of the screen, then verify that the cell moves to the top and becomes completely visible. Also confirm that a completely visible cell does not move unnecessarily.

Written by the indexing model from the issue text.