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.

TESTING.md

@@ -26,8 +26,8 @@ pass.
 
 | Folder | Contents |
 |---|---|
-| `tests/unit/` | Logic without scenes: playlist, songs, player scoring, time formatting, search, the server client |
-| `tests/integration/` | Real scenes (`MainWindow.tscn`, `SyncPopup.tscn`) run against a fake server, including the point keybinds |
+| `tests/unit/` | Logic without scenes: playlist, songs, player scoring, time formatting, search, the server client and the API classes |
+| `tests/integration/` | Real scenes (`MainWindow.tscn`, `SyncPopup.tscn`) run against a fake server, including the point keybinds and the Statistics tab |
 | `tests/fakes/` | `FakeHttpTransport`, `AutoloadSnapshot` and the `MainWindowTestSuite` base class |
 | `tests/data/` | Files the fake server sends, e.g. a short MP3 |