Split deck_mixer_controls into left and right variants.

Make 4-deck toggle switch between a 2-column and 1-column mixer control layout.
Fix some syntax errors.
Fix some damage caused by auto-formatting XML.
Make 800px the minimum width. I think we can do this!
Small square button resizing isn't working right. Switch to fixed.
Same for vertical and horizontal stretch buttons.
Add a tool bar on the top.
Move master and headphone controls to the tool bar.
Move mixer status bar controls into the tool bar.
Add a show/hide mixer toggle.
Add a library expand toggle.
Add a microphone toolbar to the status bar.
Add a sync master button.
Let rate slider grow.
Change size-policies of decks and get rid of spacers.

This arrangement prevents the decks from growing larger than they need to be.
Color the toolbar.