wlr_cursor: automatically handle layout changes

Created by: acrisci

Automatically set the cursor on outputs when a layout is attached.

Add a destroy event to wlr_output_layout and listen for it in wlr_cursor to detach the output layout cleanly.

Add a change event to wlr_output_layout and listen for it in wlr_cursor to set the cursor on the outputs in the layout.

Reconfigure the cursor on the outputs when the xcursor changes.