Sort the scripts and scenes into folders

main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.

popups/popup_backdrop.gd

@@ -0,0 +1,31 @@
+class_name PopupBackdrop
+extends ColorRect
+## Dims everything behind an open popup and keeps clicks from reaching the buttons
+## under it.
+##
+## The backdrop sits above the rest of its parent and below the popups it belongs to.
+## Popups close themselves when clicked outside, and the backdrop keeps that click from
+## reaching the buttons behind them.
+
+var _popups: Array[BasePopup] = []
+
+
+## Adds a backdrop to [param parent] and moves [param popups] above it. The backdrop
+## shows while any of them is open.
+static func add_to(parent: Control, popups: Array[BasePopup]) -> PopupBackdrop:
+	var backdrop: PopupBackdrop = PopupBackdrop.new()
+	backdrop.name = "PopupBackdrop"
+	backdrop.color = Color(0, 0, 0, 0.55)
+	backdrop.mouse_filter = Control.MOUSE_FILTER_STOP
+	backdrop._popups = popups
+	parent.add_child(backdrop)
+	backdrop.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
+	for popup: BasePopup in popups:
+		parent.move_child(popup, -1)
+		popup.visibility_changed.connect(backdrop._update)
+	backdrop._update()
+	return backdrop
+
+
+func _update() -> void:
+	visible = _popups.any(func(popup: BasePopup) -> bool: return popup.visible)