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.

light_gray_theme.tres

@@ -0,0 +1,7 @@
+[gd_resource type="Theme" load_steps=2 format=3 uid="uid://rxexo3ur85as"]
+
+[ext_resource type="StyleBox" uid="uid://blf3g88mugovr" path="res://neutral_gray.tres" id="1_alhp2"]
+
+[resource]
+Label/styles/normal = ExtResource("1_alhp2")
+PanelContainer/styles/panel = ExtResource("1_alhp2")