Fill the song cache when the settings menu is closed

Raising the cache size used to have no effect until the next song was
played. Downloads now start when the settings window closes, so clicking
the number up and down while it is open does not start any downloads.

tests/integration/test_main_window.gd

@@ -158,3 +158,59 @@ func test_cache_keeps_filling_after_a_failed_download() -> void:
 
     assert_int(Playlist.get_preloaded_songs()).is_equal(3)
     assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func _open_settings() -> void:
+    _press("SettingsButton")
+
+
+func _close_settings() -> void:
+    (_window.get_node("SettingsWindow") as Control).visible = false
+
+
+func _press_settings_button(unique_name: String) -> void:
+    (_window.get_node("SettingsWindow").get_node("%" + unique_name) as Button).pressed.emit()
+
+
+func test_raising_cache_size_does_not_download_while_settings_are_open() -> void:
+    var downloads_before: int = _server.count("/music/rand")
+    _open_settings()
+
+    _press_settings_button("IncreaseCacheButton")
+    _press_settings_button("IncreaseCacheButton")
+
+    assert_int(Playlist.number_of_tracks_to_preload).is_equal(3)
+    assert_int(_server.count("/music/rand")).is_equal(downloads_before)
+
+
+func test_closing_settings_fills_cache_to_new_size() -> void:
+    _open_settings()
+    _press_settings_button("IncreaseCacheButton")
+    _press_settings_button("IncreaseCacheButton")
+
+    _close_settings()
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+
+
+func test_changing_cache_size_back_and_forth_downloads_only_what_is_needed() -> void:
+    var downloads_before: int = _server.count("/music/rand")
+    _open_settings()
+    for i: int in 5:
+        _press_settings_button("IncreaseCacheButton")
+        _press_settings_button("LowerCacheButton")
+    _press_settings_button("IncreaseCacheButton")
+
+    _close_settings()
+
+    assert_int(_server.count("/music/rand") - downloads_before).is_equal(1)
+    assert_int(Playlist.get_preloaded_songs()).is_equal(2)
+
+
+func test_closing_settings_with_full_cache_downloads_nothing() -> void:
+    var downloads_before: int = _server.count("/music/rand")
+    _open_settings()
+
+    _close_settings()
+
+    assert_int(_server.count("/music/rand")).is_equal(downloads_before)