feat(app,android,ios): swipe gestures, touch lock and sleep timer

On Android and iOS a swipe up or down on the left half of the picture
sets the brightness and on the right half the volume; a sideways swipe
seeks, showing where it will land and seeking only when the finger
lifts. Swipes that start at the top or bottom edge are left to the
system. Android turns the device's media volume; iOS lets no app do
that, so there the swipe turns the player's own, as its slider does.
Brightness set by a swipe is handed back when the player goes.

Lock touch, in the player's menu, makes the picture ignore touches and
the back gesture; a tap brings up Unlock for a few seconds.

The sleep timer pauses after 30 or 60 minutes, or stops at the end of
the episode playing instead of going on to the next, with no up-next
card. On the desktop it is a panel like the others, behind a moon
button that shows what is left; on Android and iOS it lives with the
player host, so it keeps running with the screen off.

docs/MOBILE.md

@@ -31,8 +31,15 @@ your Drive**. See [Account and sync](ACCOUNT_AND_SYNC.md).
 - Tap to show or hide the controls.
 - **Double-tap** the left or right third of the screen to go back or forward 10 seconds. Double-tap
   the middle to pause.
-- The menu has **Episodes**, **Chapters**, **Playback speed**, the audio and subtitle tracks, and
-  **Lock to landscape**.
+- **Swipe** up and down on the left half for brightness and on the right half for volume. Swipe
+  sideways to seek: the bubble shows where you will land, and the seek happens when you let go.
+  Swipes that start at the very top or bottom of the screen are left to the system.
+- The menu has **Episodes**, **Chapters**, **Playback speed**, the audio and subtitle tracks,
+  **Lock to landscape**, **Sleep timer** and **Lock touch**.
+- **Sleep timer** pauses after 30 or 60 minutes, or stops at the end of this episode instead of
+  going on to the next. It keeps running with the screen off.
+- **Lock touch** makes the picture ignore touches, for a phone in a pocket or a child's hands. Tap
+  to bring up **Unlock**.
 - **Skip opening** and **Skip ending** work as on the desktop, AniSkip included: see
   [The player](USER_GUIDE.md#the-player).
 - Sound keeps playing with the screen off or the app in the background, with controls on the lock