Fix dec7ff6b0c: Dropdowns couldn't be closed by pressing the parent button. (#10954)

Since dropdowns self-close, the detection of re-clicking a dropdown
button no longer worked, as the dropdown is already closed.

Instead set (and then test) a flag on the parent widget to indicate that
the dropdown closed. This method avoids looping windows on every click.

src/widgets/dropdown_func.h

@@ -15,7 +15,4 @@
 /* Show drop down menu containing a fixed list of strings */
 void ShowDropDownMenu(Window *w, const StringID *strings, int selected, int button, uint32 disabled_mask, uint32 hidden_mask, uint width = 0);
 
-/* Hide drop down menu of a parent window */
-int HideDropDownMenu(Window *pw);
-
 #endif /* WIDGETS_DROPDOWN_FUNC_H */