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.

base_popup.gd

@@ -0,0 +1,37 @@
+class_name BasePopup
+extends Control
+## A window shown on top of the main window, such as Settings or About.
+##
+## Clicking outside [member panel] closes it. Popups that need to prepare or clean up
+## override [method open] or [method close] and call [code]super()[/code].
+
+## The visible part of the popup. Clicks outside it close the popup.
+@export var panel: Control
+
+
+## Shows the popup.
+func open() -> void:
+	visible = true
+
+
+## Hides the popup.
+func close() -> void:
+	visible = false
+
+
+## Opens the popup if it is closed and closes it if it is open.
+func toggle() -> void:
+	if visible:
+		close()
+	else:
+		open()
+
+
+func _input(event: InputEvent) -> void:
+	if !visible || !(event is InputEventMouseButton) || !event.pressed:
+		return
+	var click: InputEventMouseButton = panel.make_input_local(event)
+	if !Rect2(Vector2.ZERO, panel.size).has_point(click.position):
+		close()
+		# The click only closes the popup; it must not also press what is behind it
+		get_viewport().set_input_as_handled()