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.

VolumeSlider.tscn

@@ -0,0 +1,10 @@
+[gd_scene load_steps=2 format=3 uid="uid://w400rnew7453"]
+
+[ext_resource type="Script" uid="uid://bgviu7smansj1" path="res://volume_slider.gd" id="1_1aufb"]
+
+[node name="VolumeSlider" type="HSlider"]
+offset_right = 8.0
+offset_bottom = 16.0
+max_value = 1.0
+step = 0.001
+script = ExtResource("1_1aufb")