Codechange: Move dropdown and slider out of widgets directory. (#12403)

Also shuffle headers to place widget includes near end.

This leaves the widgets directory solely for defining Widget IDs.

src/genworld_gui.cpp

@@ -18,8 +18,8 @@
 #include "sound_func.h"
 #include "fios.h"
 #include "string_func.h"
-#include "widgets/dropdown_type.h"
-#include "widgets/dropdown_func.h"
+#include "dropdown_type.h"
+#include "dropdown_func.h"
 #include "querystring_gui.h"
 #include "town.h"
 #include "core/geometry_func.hpp"