This MR moves the stream list box to a separate window and it also moves the output volume slider in the "Output" section.
This also include a re-implementation of how the stream rows are created. It now uses a GListStore
(with a filter and a sorter) to store all the GvcMixerStream
s and that is bound to the GtkListBox
via bind_model. This is needed to be able to store all the stream objects before the streams window is even created.
Fixes #501 (closed) and #1303 (closed).