macosx: work-around a corner case and disallow the sidebar's library item to collapse

(cherry picked from commit 74c8629c42ef2c9bf8f96ff9caae75968f386bf5)

modules/gui/macosx/MainWindow.m

@@ -2013,6 +2013,9 @@ static VLCMainWindow *_o_sharedInstance = nil;
 /* taken under BSD-new from the PXSourceList sample project, adapted for VLC */
 - (BOOL)sourceList:(PXSourceList*)aSourceList isGroupAlwaysExpanded:(id)group
 {
+    if ([[group identifier] isEqualToString:@"library"])
+        return YES;
+
     return NO;
 }