Change: Add window description flag to disallow interactive window closing. (#11008)

This replaces/simplifies testing for a closebox to allow closing a window with right-click, and testing for specific window classes when closing all windows by hotkey.

This allows right-click closing of dropdowns and the high-score window.

src/window_gui.h

@@ -138,6 +138,7 @@ enum WindowDefaultFlag {
 	WDF_CONSTRUCTION    =   1 << 0, ///< This window is used for construction; close it whenever changing company.
 	WDF_MODAL           =   1 << 1, ///< The window is a modal child of some other window, meaning the parent is 'inactive'
 	WDF_NO_FOCUS        =   1 << 2, ///< This window won't get focus/make any other window lose focus when click
+	WDF_NO_CLOSE        =   1 << 3, ///< This window can't be interactively closed
 };
 
 /**