Hide channel combo boxes

Right now I find the combo box below the main image window a bit confusing because it duplicates information that is above the image window (the names of all the channels and a way to switch between them).  It also is confusing because it's next to the button(s) to invoke/switch between the plugins, but it has nothing conceptually to do with the plugins.  

So this PR rectifies the situation by just hiding the combo box.  I originally tried to just remove it, but this lead to errors because it seems that the channels were not getting correctly initialized for some reason.

I also could only test the qt version, because I don't have an easy way to run something with gtk right now.