Change: Use CRTP-mixins to compose dropdown list items.

This allows list items to built from component parts as required, and additional
functionality is added:

* Icons and text can be positioned at the start or end of the space (templated.)
* Font size of text can be changed (templated.)
* Palette of sprites can be set (runtime.)

src/genworld_gui.cpp

@@ -358,7 +358,7 @@ static DropDownList BuildTownNameDropDown()
 	size_t newgrf_size = list.size();
 	/* Insert newgrf_names at the top of the list */
 	if (newgrf_size > 0) {
-		list.push_back(std::make_unique<DropDownListItem>(-1, false)); // separator line
+		list.push_back(std::make_unique<DropDownListDividerItem>(-1, false)); // separator line
 		newgrf_size++;
 	}