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/autoreplace_gui.cpp

@@ -24,7 +24,7 @@
 #include "core/geometry_func.hpp"
 #include "rail_gui.h"
 #include "road_gui.h"
-#include "widgets/dropdown_func.h"
+#include "dropdown_func.h"
 #include "autoreplace_cmd.h"
 #include "group_cmd.h"
 #include "settings_cmd.h"