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/dropdown_func.h

@@ -0,0 +1,18 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file dropdown_func.h Functions related to the drop down widget. */
+
+#ifndef DROPDOWN_FUNC_H
+#define DROPDOWN_FUNC_H
+
+#include "window_gui.h"
+
+/* Show drop down menu containing a fixed list of strings */
+void ShowDropDownMenu(Window *w, const StringID *strings, int selected, WidgetID button, uint32_t disabled_mask, uint32_t hidden_mask, uint width = 0);
+
+#endif /* DROPDOWN_FUNC_H */