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.

SearchPopup.tscn

@@ -0,0 +1,52 @@
+[gd_scene format=3 uid="uid://bxydgil1yifps"]
+
+[ext_resource type="Script" uid="uid://bg5yo08638600" path="res://search_popup.gd" id="1_m7bij"]
+[ext_resource type="Theme" path="res://popup_theme.tres" id="popup_theme"]
+
+[node name="SearchPopup" type="Control" unique_id=616238617 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_m7bij")
+panel = NodePath("SearchPanel")
+
+[node name="SearchPanel" type="Panel" parent="." unique_id=1262945994]
+unique_name_in_owner = true
+layout_mode = 0
+offset_right = 1152.0
+offset_bottom = 640.0
+
+[node name="ClearButton" type="Button" parent="." unique_id=1549439398]
+layout_mode = 0
+offset_left = 936.0
+offset_top = 16.0
+offset_right = 1027.0
+offset_bottom = 56.0
+scale = Vector2(2, 2)
+focus_mode = 0
+action_mode = 0
+text = "Clear"
+
+[node name="Searchbar" type="TextEdit" parent="." unique_id=2112669372]
+layout_mode = 0
+offset_left = 32.0
+offset_top = 16.0
+offset_right = 468.0
+offset_bottom = 56.0
+scale = Vector2(2, 2)
+placeholder_text = "Search"
+
+[node name="ScrollContainer" type="ScrollContainer" parent="." unique_id=213733486]
+layout_mode = 0
+offset_top = 128.0
+offset_right = 576.0
+offset_bottom = 384.0
+scale = Vector2(2, 2)
+
+[node name="VBoxContainer" type="VBoxContainer" parent="ScrollContainer" unique_id=1567403144]
+layout_mode = 2
+size_flags_horizontal = 3