feat(app): player side panel, seek HUD and pause screen

The audio, subtitle, speed and chapter pickers were stock popup menus
hanging off the transport. They are now panels down the right of the
picture, in the place and style of the episode list: one at a time, a
whole row per choice, the chosen one filled and ticked, closed by its
button again or by Esc. The library's mini player keeps its menus.

What a key did is shown in the middle of the picture with an icon, and
the volume with a bar. Seeks the same way in quick succession add up, so
three presses of the right arrow read "+ 90 s".

After five seconds paused with the mouse still, the picture dims and the
controls give way to what is playing: the logo or the name, the episode,
and its synopsis, or the show's when the episode has none. Moving the
mouse brings the controls back.

docs/USER_GUIDE.md

@@ -139,11 +139,17 @@ 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** are chosen from the buttons at the bottom right. The app remembers your
-  choice for each show. The subtitle menu 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.
+- **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** 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.
+- **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:
+  the show's logo or name, the episode, and what it is about. Move the mouse to get the controls
+  back.
 
 ### Keyboard
 
@@ -173,7 +179,7 @@ Press **?** anywhere to see this list in the app.
 | E | The list of episodes |
 | F or double-click | Fullscreen |
 | Ctrl+S | Save the frame to `Pictures/proton-stream` |
-| Esc | Leave fullscreen, then the player |
+| Esc | Close a panel, leave fullscreen, then leave the player |
 
 Media keys and your desktop's media controls work too.