Just talking about panels with controls, forgetting whether they can be hidden or not:
Initially, panels contained controls in MIDI Designer.... this was amazing because you have COPIES of panels. But I digress.
So if you had save, load, and change channel in design mode for panels you'd be happier?
Then: I have to think of how to deal with panels in MIDI Designer. I might not touch panels at all, but rather add a concept of GROUPS in MIDI Designer (like in design programs). That would be VERY simple. Groups are what you call containers, I think...?