Add #6887: Option to show zone inside local authority boundary of towns

Can be found at town information > local authority window
Layout for button is same as Graph Keys
Turn on/off for every town individually

src/widgets/town_widget.h

@@ -25,6 +25,7 @@ enum TownDirectoryWidgets {
 /** Widgets of the #TownAuthorityWindow class. */
 enum TownAuthorityWidgets {
 	WID_TA_CAPTION,      ///< Caption of window.
+	WID_TA_ZONE_BUTTON,  ///< Turn on/off showing local authority zone.
 	WID_TA_RATING_INFO,  ///< Overview with ratings for each company.
 	WID_TA_COMMAND_LIST, ///< List of commands for the player.
 	WID_TA_SCROLLBAR,    ///< Scrollbar of the list of commands.