[qmf] When messageListModel limit changes add more messages to model ins...

...tead of full refresh.
I don't see any reason to do this step-by-step removal inside the loop?  If we want to ensure that the model does not exceed the limit during this loop, then we should probably go through the sorted indices prior to this loop to remove any that exceed the limit - then we can remove the necessary number of messages prior to performing this insertion loop.

I don't really see that as necessary, though - I would prefer to just insert all the items with valid indices, then remove any messages that exceed the limit in one step.