Bugfix: Couldn't subclass Panel
I was trying to create a subclass from Panel and came into this snag. I'm not sure if there was a reason Panel was using a hard coded _constructor method, but I changed it to be more generic. I'm new to open source, so constructive criticism is welcome. Am I doing this right?

Welcome to open source! You're on the right lines, I've just highlighted a few potential problems. Other people who're more familiar with the design of panel may give you more suggestions. Remember, although the comments might come across as quite terse, we are grateful that you're using and improving the code.
 Thanks for the feedback. I changed _constructor to return type(self), and the _constructorGroupBy thing was a consequence of sloppy search and replace, so I fixed that. Should I submit another request or keep this one?
 Merged in. Welcome to the project!

Future reference, it's easier (on you) if you work in a non-master branch. When you update, you'll have to "git reset --hard <upstream>/master"
 @joshuaar a more fundamental question is: why are you subclassing Panel? Often when people ask me about subclassing I talk them out of it (viz "favor composition over inheritance")
