Rebase the cursor after applying transactions

This approaches cursor rebasing from a different angle. Rather than
littering the codebase with cursor_rebase calls and using transaction
callbacks, this just runs cursor_rebase after applying every transaction
- but only if there's outputs connected, because otherwise it causes a
crash during shutdown.

There is one known case where we still need to call cursor_rebase
directly, and that's when running `seat seat0 cursor move ...`. This
command doesn't set anything as dirty so no transaction occurs.

include/sway/desktop/transaction.h

@@ -28,13 +28,6 @@ struct sway_view;
  */
 void transaction_commit_dirty(void);
 
-/**
- * Same as above, but runs the specific callback when the transaction is
- * applied.
- */
-void transaction_commit_dirty_with_callback(
-		void (*callback)(void *data), void *data);
-
 /**
  * Notify the transaction system that a view is ready for the new layout.
  *