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/command_type.h

@@ -295,6 +295,7 @@ enum Commands {
 	CMD_REMOVE_STORY_PAGE,            ///< remove a story page
 	CMD_REMOVE_STORY_PAGE_ELEMENT,    ///< remove a story page element
 	CMD_SCROLL_VIEWPORT,              ///< scroll main viewport of players
+	CMD_STORY_PAGE_BUTTON,            ///< selection via story page button
 
 	CMD_LEVEL_LAND,                   ///< level land