Revert "Drop down list: stop using has_minimum="false" and remove a workaround"

This reverts commit 1e8d8ff7d7.

@CelticMinstrel and @Vultraz pointed out that the change causes menus
and dropdowns (which are implemented as dropdowns) to initially have
an item selected, which isn't desired.

data/gui/window/drop_down_list.cfg

@@ -72,6 +72,7 @@ min(
 					[listbox]
 						id = "list"
 						definition = "default"
+						has_minimum = "false"
 
 						[list_definition]
 

src/gui/dialogs/drop_down_menu.cpp

@@ -61,6 +61,18 @@ namespace {
 			}
 		}
 
+		/* FIXME: This dialog uses a listbox with 'has_minimum = false'. This allows a listbox to have 0 or more selections,
+		 * and selecting the same entry toggles that entry's state (ie, if it was selected, it will be deselected). Because
+		 * of this, selecting the same entry in the dropdown list essentially sets the list's selected row to -1, causing problems.
+		 *
+		 * In order to work around this, we first manually deselect the selected entry here. This handler is called *before*
+		 * the listbox's click handler, and as such the selected item will remain toggled on when the click handler fires.
+		 */
+		const int sel = list.get_selected_row();
+		if(sel >= 0) {
+			list.select_row(sel, false);
+		}
+
 		SDL_Rect rect = window.get_rectangle();
 		if(!sdl::point_in_rect(coordinate, rect)) {
 			window.set_retval(window::CANCEL);