macosx: enable opening of *.txt named subtitles through the Video menu

modules/gui/macosx/controls.m

@@ -584,7 +584,7 @@
     [openPanel setCanChooseFiles: YES];
     [openPanel setCanChooseDirectories: NO];
     [openPanel setAllowsMultipleSelection: YES];
-    i_returnValue = [openPanel runModalForDirectory: [NSString stringWithUTF8String: path] file: nil types: [NSArray arrayWithObjects: @"cdg",@"@idx",@"srt",@"sub",@"utf",@"ass",@"ssa",@"aqt",@"jss",@"psb",@"rt",@"smi", nil]];
+    i_returnValue = [openPanel runModalForDirectory: [NSString stringWithUTF8String: path] file: nil types: [NSArray arrayWithObjects: @"cdg",@"@idx",@"srt",@"sub",@"utf",@"ass",@"ssa",@"aqt",@"jss",@"psb",@"rt",@"smi",@"txt", nil]];
     free( path );
 
     if( i_returnValue == NSOKButton )