Cursor: add on-focus-change option

Warp the cursor to the center of the focused view if the cursor is not
in the bounding box of that view already. This helps the user to keep
track of their cursor when they mostly use the keyboard and the cursor
becomes hidden most of the time, also helps trackpad/trackpoint users.

completions/bash/riverctl

@@ -64,7 +64,7 @@ function __riverctl_completion ()
 			"unmap") OPTS="-release" ;;
 			"attach-mode") OPTS="top bottom" ;;
 			"focus-follows-cursor") OPTS="disabled normal always" ;;
-			"set-cursor-warp") OPTS="disabled on-output-change" ;;
+			"set-cursor-warp") OPTS="disabled on-output-change on-focus-change" ;;
 			"hide-cursor") OPTS="timeout when-typing" ;;
 			*) return ;;
 		esac