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.

popup_backdrop.gd

@@ -7,12 +7,12 @@ extends ColorRect
 ## Popups close themselves when clicked outside, and the backdrop keeps that click from
 ## reaching the buttons behind them.
 
-var _popups: Array[PopupWindow] = []
+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[PopupWindow]) -> PopupBackdrop:
+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)
@@ -20,7 +20,7 @@ static func add_to(parent: Control, popups: Array[PopupWindow]) -> PopupBackdrop
 	backdrop._popups = popups
 	parent.add_child(backdrop)
 	backdrop.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
-	for popup: PopupWindow in popups:
+	for popup: BasePopup in popups:
 		parent.move_child(popup, -1)
 		popup.visibility_changed.connect(backdrop._update)
 	backdrop._update()
@@ -28,4 +28,4 @@ static func add_to(parent: Control, popups: Array[PopupWindow]) -> PopupBackdrop
 
 
 func _update() -> void:
-	visible = _popups.any(func(popup: PopupWindow) -> bool: return popup.visible)
+	visible = _popups.any(func(popup: BasePopup) -> bool: return popup.visible)