tiling: return the newly created tiles in split()

This new return value is mainly intended for usage trough scripting.

In scripting there was no safe way to know which were the two "new" tiles after splitting, because two things can happen: either two new child tiles are created or if the layout direction agrees with the current layout, a new sibling is created, so we can't rely on tile.tiles.

This makes it return an Array containing the result of the split, making scripts much cleaner and with less heuristics.