[Feature Request] Disable tiles in layout / Free space

When editing the layout, allow disabling individual tiles.

Disabled tiles will not allow windows to be dragged into / take the space. Neither manually, nor automatically.

Start by tracing the layout editor and the logic that lets windows be dragged into or automatically assigned to tiles. Define disabled-tile behavior for both manual and automatic placement, then verify that disabled tiles reject windows and leave their space unused.

Written by the indexing model from the issue text.