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/USER_GUIDE.md

@@ -140,12 +140,15 @@ The picture plays inside the app's window. Move the mouse to show the controls.
   credits** in Settings to skip them without asking.
 - **Up next** counts down 10 seconds when the credits start, then plays the next episode. Choose
   **Play now**, **Cancel** or **Watch till the end**.
-- **Subtitles and audio**, **Speed** and **Chapters** are buttons at the bottom right. Each opens
-  a panel down the right of the picture; click the button again, or press Esc, to close it.
+- **Subtitles and audio**, **Speed**, **Chapters** and **Sleep timer** are buttons at the bottom
+  right. Each opens a panel down the right of the picture; click the button again, or press Esc,
+  to close it.
 - **Subtitles and audio** remembers your choice for each show. The panel also has **Size** and
   **Position**, which are saved like the ones in Settings and change styled subtitles only when
   they are restyled, and **Delay**, which moves subtitles earlier or later for this file only.
 - **Speed** goes from 0.5× to 3×, and stays for the next episode.
+- **Sleep timer**, the moon, pauses after 30 or 60 minutes, or stops at the end of this episode
+  instead of going on to the next.
 - **Seeking and volume** show what they did in the middle of the picture. Several presses of ← or
   → add up, so three presses of → say "+ 90 s".
 - **Paused** for 5 seconds with the mouse still, the picture dims and shows what you are watching: