Expose Control::_get_drag_data() as non-const function

doc/classes/Control.xml

@@ -75,7 +75,7 @@
 				[/codeblocks]
 			</description>
 		</method>
-		<method name="_get_drag_data" qualifiers="virtual const">
+		<method name="_get_drag_data" qualifiers="virtual">
 			<return type="Variant" />
 			<param index="0" name="at_position" type="Vector2" />
 			<description>

scene/gui/control.h

@@ -349,7 +349,7 @@ protected:
 	GDVIRTUAL0RC(Vector2, _get_minimum_size)
 	GDVIRTUAL1RC(String, _get_tooltip, Vector2)
 
-	GDVIRTUAL1RC(Variant, _get_drag_data, Vector2)
+	GDVIRTUAL1R(Variant, _get_drag_data, Vector2)
 	GDVIRTUAL2RC(bool, _can_drop_data, Vector2, Variant)
 	GDVIRTUAL2(_drop_data, Vector2, Variant)
 	GDVIRTUAL1RC(Object *, _make_custom_tooltip, String)