Expose node.select exec callback to an operator

Internally it was only invoke callback set for an
operator template. This invoke was setting such
properties as mouse_x and mouse_y and was calling
an exec function.

This meant that t seemed to be really easy to
use node.select operator from by giving a mouse
positions, but in fact it wasn't possible (because
it requires exec callback)

This commit sets operator's template exec callback,
which makes it possible using node.select from
python.

source/blender/editors/space_node/node_select.c

@@ -429,6 +429,7 @@ void NODE_OT_select(wmOperatorType *ot)
 	
 	/* api callbacks */
 	ot->invoke = node_select_invoke;
+	ot->exec = node_select_exec;
 	ot->poll = ED_operator_node_active;
 	
 	/* flags */