Add a Statistics tab with a card for every soundtrack

Part of #41. The top bar has Quiz and Statistics tabs instead of the
Statistics button. The Statistics tab replaces the quiz below the top bar
(the song keeps playing) and shows every soundtrack as a card with its
plays, last played date and most and least played songs, loaded from the
new GET /api/v1/statistics/soundtracks. The cards can be searched (with
the Search popup's matching, now in SoundtrackSearch), filtered on played
or never played, and sorted A-Z, Z-A, most or least played. Clicking a
card opens a popup with all its songs.

Screenshots of the other states only differ in the top bar, and the
layout baseline is taken again with the new statistics states.

themes/README.md

@@ -42,8 +42,9 @@ inspector or `theme_type_variation` in code:
 | `TextMedium` | Label | Rows in the sync result (18) |
 | `HighlightLarge`, `HighlightMedium` | Label | Orange text: sync headers and every other sync row |
 | `SongRowLabel`, `SongRowLabelPlaying` | Label | A song's text, and the one that is playing |
-
-The **PopupPanel** type itself is styled too, for the Statistics popup.
+| `TabButton` | Button | The Quiz and Statistics tabs; the chosen one is orange |
+| `StatisticsCard`, `StatisticsCardHover` | PanelContainer | A soundtrack card in the statistics, and the one under the mouse |
+| `StatisticsGrid` | GridContainer | The grid of soundtrack cards, with the space between them |
 
 **Palette**: colours used from code.