This does a few small things
-Moves buttons down 20px to the same height as the list boxes
-Adds a QFrame around scrollArea for mixer list.
Q: Why was this done?
A: When you go to style the mixer list in regards to adding a border,
shadow, or glow, it needs to be done on the QFrame. If you do it on the
scrollArea itself, the scrollbars will overlap the bottom of the border,
causing the border to look cut-off. Additionally, the other two sources
and scenes list widgets already had frames, so they did not have this
problem.
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