Feature: Push-buttons on storybook pages (#7896)

Allow more direct player-initiated interaction for Game Scripts, by letting the GS put push-buttons on storybook pages. These buttons can either trigger an immediate event, or require the player to first select a tile on the map, or a vehicle.

Additionally this reworks how the storybook pages are layouted and rendered, to allow for slightly more complex layouts, and maybe speeding drawing up a bit.

src/script/api/script_event.hpp

@@ -54,6 +54,9 @@ public:
 		ET_EXCLUSIVE_TRANSPORT_RIGHTS,
 		ET_ROAD_RECONSTRUCTION,
 		ET_VEHICLE_AUTOREPLACED,
+		ET_STORYPAGE_BUTTON_CLICK,
+		ET_STORYPAGE_TILE_SELECT,
+		ET_STORYPAGE_VEHICLE_SELECT,
 	};
 
 	/**