channel_sidebar: redesign search
The current design was trying too much to be smart and keyboard centric and stuff, this is a much simpler and better design that is inline with what GNOME apps generally use.
We needed a separate search stack child because our nested expanders don't really work well there.
I have also noticed that the channel sidebar brings quite a performance hit (focusing/defocusing), this is probbaly because of how many listboxes we use.
Closes #27 (closed)