Disco! 3.5.1

One new setting: the screen can lock when it times out.

Lock on timeout

Settings › Battery & power › Lock on timeout, Off by default.

• On: when the screen turns off by itself, it locks like the power button. Touches in a pocket can't wake it;
  one press of the power button does, at your brightness.
• Off: as before. A tap wakes the screen.

The lock comes only when the screen turns fully off. While the screensaver shows, a tap still wakes it.

It also fixes the power button after a timeout (#18). The player only knew about its own presses, so after the
screen turned off by itself the next press locked instead of waking, and getting the screen back took button,
button, tap. With Lock on timeout On, the player locks itself at the timeout, so one press wakes the screen.

Thanks to @dickofn, whose investigation and measurements in #19 this builds on.

Binary record: RELEASE_3.5.1.md. Update from the SD card as usual
(guide); no reflash.