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.

sway/tree/view.c

@@ -661,7 +661,6 @@ void view_unmap(struct sway_view *view) {
 				cursor_warp_to_workspace(seat->cursor, node->sway_workspace);
 			}
 		}
-		cursor_rebase(seat->cursor);
 	}
 
 	transaction_commit_dirty();

sway/tree/workspace.c

@@ -404,7 +404,6 @@ bool workspace_switch(struct sway_workspace *workspace,
 	}
 	seat_set_focus(seat, next);
 	arrange_workspace(workspace);
-	cursor_rebase(seat->cursor);
 	return true;
 }