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.

players_panel.gd

@@ -13,9 +13,9 @@ signal player_added(player_object: PlayerObject)
 signal player_removed(player_object: PlayerObject)
 
 ## Emitted when the character button in [param row] is pressed.
-signal change_character_requested(row: Player, player_name: String)
+signal change_character_requested(row: PlayerRow, player_name: String)
 
-var _player_row: PackedScene = preload("res://Player.tscn")
+var _player_row: PackedScene = preload("res://PlayerRow.tscn")
 
 @onready
 var add_players_button: TextureButton = $VBoxContainer/HBoxContainer/AddPlayersButton
@@ -68,7 +68,7 @@ func reload() -> void:
 	for player_object: PlayerObject in Settings.player_array:
 		GameLog.debug(player_object.player_name)
 
-		var new_player: Player = _player_row.instantiate()
+		var new_player: PlayerRow = _player_row.instantiate()
 		new_player.player_object = player_object
 		new_player.list_index = counter
 
@@ -79,7 +79,7 @@ func reload() -> void:
 		counter += 1
 
 
-func _on_change_character_clicked(row: Player, player_name: String) -> void:
+func _on_change_character_clicked(row: PlayerRow, player_name: String) -> void:
 	change_character_requested.emit(row, player_name)