Handling pad removing

[Karolk99]

Right now there is no handle_pad_removed callback in compositor_element.ex. This can cause problems when we get handle_pad_added without handle_start_of_stream in this scenario we won't always get "end_of stream" message.
That's why we have to implement 'handle_pad_removed` to be sure that we handle all cases of removing pads.

Also there is another problem with a stream that was collected but not mixed. If compositor_element get "end_of_stream" from certain pad it won't try to mix frames that were blocked by this pad so if we don't get any new buffers we will never mix those saved frames. My suggestion to fix that is to invoke some function that will try to mix as much frames as it can in two callbacks: handle_pad_removed and handle_end_of_stream