After $PutModel and $GetModel return to the Main Thread

[smaugho]

Right now, after a $PutModel or $GetModel is executed, if it was executed in the background, then if you want to make any change in the UI, need to implicitly write the action $UIThread, for instance:

$PutModel(myModel);
$UIThread();
//Do something here in the UI.

But $PutModel knows at priori if the action is executed in the background or not, so, it would be nice to invoke the next runnable in the list of actions already in the UIThread avoiding the errors that would occurs if the programmer doesn't write the $UIThread annotation.