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.

ShortcutsPopup.tscn

@@ -0,0 +1,50 @@
+[gd_scene load_steps=2 format=3 uid="uid://bd1by80q1v27e"]
+
+[ext_resource type="Script" uid="uid://rolypd0cqd6h" path="res://shortcuts_popup.gd" id="1_12pxi"]
+[ext_resource type="Theme" path="res://popup_theme.tres" id="popup_theme"]
+
+[node name="ShortcutsPopup" type="Control" node_paths=PackedStringArray("panel")]
+theme = ExtResource("popup_theme")
+custom_minimum_size = Vector2(700, 700)
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("1_12pxi")
+panel = NodePath("ShortcutsPanel")
+
+[node name="ShortcutsPanel" type="Panel" parent="."]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(700, 700)
+layout_mode = 0
+offset_right = 40.0
+offset_bottom = 40.0
+
+[node name="ScrollContainer" type="ScrollContainer" parent="ShortcutsPanel"]
+custom_minimum_size = Vector2(700, 700)
+layout_mode = 0
+offset_right = 40.0
+offset_bottom = 40.0
+
+[node name="shortcutsVBox" type="VBoxContainer" parent="ShortcutsPanel/ScrollContainer"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(700, 700)
+layout_mode = 2
+
+[node name="HBoxContainer" type="HBoxContainer" parent="ShortcutsPanel/ScrollContainer/shortcutsVBox"]
+layout_mode = 2
+alignment = 1
+
+[node name="Label" type="Label" parent="ShortcutsPanel/ScrollContainer/shortcutsVBox/HBoxContainer"]
+layout_mode = 2
+size_flags_horizontal = 3
+text = "testar"
+horizontal_alignment = 1
+
+[node name="Label2" type="Label" parent="ShortcutsPanel/ScrollContainer/shortcutsVBox/HBoxContainer"]
+layout_mode = 2
+size_flags_horizontal = 3
+text = "testera 2"
+horizontal_alignment = 1