Show every song in the statistics, as an alternative to the cards

A Soundtracks / Songs switch in the statistics toolbar shows a list of
every song with its soundtrack and how often it was played. It is searched
on both song and soundtrack names, filtered and sorted like the cards, and
clicking a song shows its soundtrack's popup.

The server has tens of thousands of songs, so the list only makes the rows
on screen and reuses them while scrolling, songs are sorted once per load,
and searching waits for a pause in typing.

Closes #47

statistics/SongStatsList.tscn

@@ -0,0 +1,68 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://statistics/song_stats_list.gd" id="list_script"]
+
+[node name="SongStatsList" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+script = ExtResource("list_script")
+
+[node name="Layout" type="HBoxContainer" parent="."]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+
+[node name="Columns" type="VBoxContainer" parent="Layout"]
+layout_mode = 2
+size_flags_horizontal = 3
+mouse_filter = 2
+
+[node name="Header" type="PanelContainer" parent="Layout/Columns"]
+layout_mode = 2
+mouse_filter = 2
+theme_type_variation = &"SongStatsHeader"
+
+[node name="Titles" type="HBoxContainer" parent="Layout/Columns/Header"]
+layout_mode = 2
+mouse_filter = 2
+theme_override_constants/separation = 16
+
+[node name="SongTitle" type="Label" parent="Layout/Columns/Header/Titles"]
+layout_mode = 2
+size_flags_horizontal = 3
+size_flags_stretch_ratio = 3.0
+theme_type_variation = &"HighlightMedium"
+text = "Song"
+
+[node name="SoundtrackTitle" type="Label" parent="Layout/Columns/Header/Titles"]
+layout_mode = 2
+size_flags_horizontal = 3
+size_flags_stretch_ratio = 2.0
+theme_type_variation = &"HighlightMedium"
+text = "Soundtrack"
+
+[node name="PlayedTitle" type="Label" parent="Layout/Columns/Header/Titles"]
+custom_minimum_size = Vector2(100, 0)
+layout_mode = 2
+theme_type_variation = &"HighlightMedium"
+text = "Played"
+horizontal_alignment = 2
+
+[node name="RowsArea" type="Control" parent="Layout/Columns"]
+unique_name_in_owner = true
+clip_contents = true
+layout_mode = 2
+size_flags_vertical = 3
+
+[node name="ScrollBar" type="VScrollBar" parent="Layout"]
+unique_name_in_owner = true
+layout_mode = 2