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.

tests/integration/test_scene_split.gd

@@ -6,7 +6,7 @@ extends MainWindowTestSuite
 
 const MAIN_WINDOW_CHILDREN: Array[String] = [
 	"Background", "TopBar", "Quiz", "PopupBackdrop", "SearchPopup", "SyncPopup", "QRPopup",
-	"ShortcutsPopup", "StatisticsPopupPanel", "AboutPopup", "LogPopup",
+	"ShortcutsPopup", "StatisticsView", "AboutPopup", "LogPopup", "SoundtrackStatsPopup",
 ]
 
 ## The parts of the quiz and the scene each one comes from.
@@ -27,15 +27,14 @@ const LOCKED_BY_SYNC: Array[String] = [
 ]
 
 const TOP_BAR_CHILDREN: Array[String] = [
-	"TopPanelContainer", "Title", "SyncButton", "SearchButton", "StatisticsButton",
-	"ShortcutsButton", "AboutButton", "QRButton",
+	"TopPanelContainer", "Title", "SyncButton", "SearchButton", "ShortcutsButton",
+	"AboutButton", "QRButton", "QuizTab", "StatisticsTab",
 ]
 
 ## Each top bar button and the popup it opens in the main window.
 const TOP_BAR_POPUPS: Dictionary = {
 	"SearchButton": "SearchPopup",
 	"SyncButton": "SyncPopup",
-	"StatisticsButton": "StatisticsPopupPanel",
 	"ShortcutsButton": "ShortcutsPopup",
 	"AboutButton": "AboutPopup",
 	"QRButton": "QRPopup",
@@ -285,10 +284,3 @@ func test_alt_enter_switches_fullscreen_and_settings_shows_it() -> void:
 	assert_bool(settings.fullscreen_button.button_pressed).is_equal(!was_fullscreen)
 	await _press_keys([KEY_ALT, KEY_ENTER])
 	assert_bool(Settings.fullscreen).is_equal(was_fullscreen)
-
-
-func test_statistics_counts_the_soundtracks() -> void:
-	_top_bar_button("StatisticsButton").pressed.emit()
-
-	assert_str((_window.get_node("StatisticsPopupPanel/StatisticsLabel") as Label).text) \
-			.is_equal("Total amount of soundtracks in the playlist: 2")