Support terminal focus notifications

Support terminal focus notifications

* lisp/frame.el (handle-focus-in,handle-focus-out): Make event
argument optional.
(blink-cursor-check): Make sure that the current frame is a
window-system frame before restarting the blink timer. TTY frames
can get focus, but don't need a blink timer because the terminal
will do the blinking.

* lisp/term/xterm.el
(xterm-handle-focus-in,xterm-handle-focus-out): New functions.
(xterm-rxvt-function-map): Recognize focus notification sequences.
(xterm--init-focus-tracking): New function.
(terminal-init-xterm): Call it.