dragicon: Make a function propertly exported

Mark gtk_drag_icon_create_widget_for_value with
GDK_AVAILABLE_IN_ALL, since it is meant to be public.
Also, clarify the documentation a bit.

Fixes: #3325

gtk/gtkdragicon.c

@@ -500,6 +500,7 @@ gtk_drag_icon_get_child (GtkDragIcon *self)
  * Creates a widget that can be used as a drag icon for the given
  * @value.
  *
+ * Supported types include strings, #GdkRGBA and #GtkTextBuffer.
  * If GTK does not know how to create a widget for a given value,
  * it will return %NULL.
  *

gtk/gtkdragicon.h

@@ -51,6 +51,7 @@ void            gtk_drag_icon_set_from_paintable (GdkDrag      *drag,
                                                   int           hot_x,
                                                   int           hot_y);
 
+GDK_AVAILABLE_IN_ALL
 GtkWidget *     gtk_drag_icon_create_widget_for_value           (const GValue           *value);
 
 G_END_DECLS