macosx: auto expand when we select one of the elements in the preferences view.

This will hopefully help avoid some of the reports where people fail to notice that there is an expand triangle next to what they clicked on.

modules/gui/macosx/prefs.m

@@ -261,6 +261,7 @@ static VLCPrefs *_o_sharedMainInstance = nil;
 - (void)outlineViewSelectionDidChange:(NSNotification *)o_notification
 {
     [[o_tree itemAtRow:[o_tree selectedRow]] showView: o_prefs_view];
+    [o_tree expandItem:[o_tree itemAtRow:[o_tree selectedRow]]];
 }
 
 @end