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.

character_select_popup.gd

@@ -0,0 +1,66 @@
+class_name CharacterSelectPopup
+extends BasePopup
+
+@onready
+var character_panel: Panel = %CharacterPanel
+
+@onready
+var title_label: Label = %TitleLabel
+
+@onready
+var character_grid: GridContainer = %CharacterGridContainer
+
+@onready
+var close_timer: Timer = %CloseTimer
+
+var can_select_character: bool = false
+
+signal new_new_character_selected(file_name: String)
+
+var character: PackedScene = preload("res://CharacterCard.tscn")
+
+func _ready() -> void:
+	close_timer.timeout.connect(_on_close_timeout)
+
+func _on_close_timeout() -> void:
+	GameLog.debug("_on_close_timeout!")
+	Settings.character_select_open = false
+	close_timer.stop()
+
+func load_characters() -> void:
+	GameLog.debug("load_characters")
+	var fetch_character_list: Callable = func fetch_character_list(list) -> void:
+		GameLog.debug("fetch_character_list")
+		Settings.delete_children(character_grid)
+		if typeof(list) == TYPE_ARRAY:
+			for character_name: String in list:
+				GameLog.debug("character_name ", character_name)
+				var character_box: CharacterCard = character.instantiate()
+				character_grid.add_child(character_box)
+				character_box.custom_minimum_size = Vector2(235, 145)
+				character_box.size = Vector2(235, 145)
+				character_box.set_character_name(character_name)
+				character_box.connect("new_character_selected", _on_character_selected)
+	MusicServerClient.fetch_character_list(fetch_character_list)
+
+func show_grid(player_name: String) -> void:
+	GameLog.debug("show_grid")
+	title_label.text = "Select character for " + player_name
+	close_timer.stop()
+	open()
+	can_select_character = true
+	Settings.character_select_open = true
+
+func close() -> void:
+	super()
+	Settings.character_select_open = false
+
+func _on_character_selected(texture: ImageTexture) -> void:
+	GameLog.debug("_on_character_selected")
+	# Not close(): character_select_open stays true until close_timer fires, so the click
+	# that picked the character does not also select a song behind it
+	self.visible = false
+	if can_select_character:
+		new_new_character_selected.emit(texture)
+		close_timer.start()
+		can_select_character = false