gridview: Redo tile management

gridview: Redo tile management

Instead of the custom size property, use the new tile size.

Also introduce the ability to split tiles, so that gridview can split a
layout that would look like (question mark denoting cells without a
widget, which in this case would be a single tile)

█ █ █ ? ?
? ? ? ? ?
? ? ? ? ?
? ? ?

into 3 rectangular tiles like so:

█ █ █ A A
B B B B B
B B B B B
C C C

This of course also means we need to be able to merge those tiles again
when cells got added/deleted or the gridview was resized. For that job,
gtk_list_tile_gc() exists now, which removes tiles without items and
merges adjacent tiles without widgets.