Name popups *Popup and give scripts, scenes and classes matching names

Every popup is now called *Popup instead of *Window in its file, root node,
node name, class and variable, and PopupWindow becomes BasePopup. Player is
PlayerRow, SongListObject is SongRow and Character is CharacterCard. Script
files are snake_case and scene files PascalCase, each matching its class or
root node, and LightGrayTheme.tres is light_gray_theme.tres.

WinnerPopup.tscn

@@ -0,0 +1,56 @@
+[gd_scene format=3 uid="uid://cxlsmxnrp0agh"]
+
+[ext_resource type="Script" uid="uid://dxpb22xbwf3t1" path="res://winner_popup.gd" id="1_winner_script"]
+[ext_resource type="Texture2D" uid="uid://r4as0nmtoa7p" path="res://noCharacter.png" id="2_no_character"]
+[ext_resource type="Theme" path="res://popup_theme.tres" id="popup_theme"]
+
+[sub_resource type="LabelSettings" id="LabelSettings_hr75l"]
+font_size = 35
+
+[node name="WinnerPopup" type="Control" unique_id=1111111111 node_paths=PackedStringArray("panel")]
+theme = ExtResource("popup_theme")
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("1_winner_script")
+panel = NodePath("WinnerPanel")
+
+[node name="WinnerPanel" type="Panel" parent="." unique_id=89212869]
+unique_name_in_owner = true
+layout_mode = 0
+offset_right = 500.0
+offset_bottom = 600.0
+
+[node name="VBoxContainer" type="VBoxContainer" parent="WinnerPanel" unique_id=471159349]
+layout_mode = 0
+offset_top = 25.0
+offset_right = 500.0
+offset_bottom = 282.0
+size_flags_horizontal = 3
+size_flags_vertical = 3
+alignment = 1
+
+[node name="WinnerPicture" type="TextureRect" parent="WinnerPanel/VBoxContainer" unique_id=1543774067]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(480, 200)
+layout_mode = 2
+texture = ExtResource("2_no_character")
+expand_mode = 3
+stretch_mode = 5
+
+[node name="WinnerLabel" type="Label" parent="WinnerPanel/VBoxContainer" unique_id=1437597286]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Winner!"
+label_settings = SubResource("LabelSettings_hr75l")
+horizontal_alignment = 1
+vertical_alignment = 2
+
+[node name="PlayersList" type="VBoxContainer" parent="WinnerPanel/VBoxContainer" unique_id=471159350]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+size_flags_vertical = 3