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/company_gui.cpp

@@ -23,7 +23,7 @@
 #include "company_manager_face.h"
 #include "strings_func.h"
 #include "timer/timer_game_economy.h"
-#include "widgets/dropdown_type.h"
+#include "dropdown_type.h"
 #include "tilehighlight_func.h"
 #include "company_base.h"
 #include "core/geometry_func.hpp"