Improve mixer matrix UX

[klausman]

So the skeuomorphic UI of the mixer setup is nice and compact but a pain to use if you're messing with it often and want to match gain/levels on more than one channel (think stereo sets). It's even worse when you have to use it with a trackpad or trackpoint on a laptop, since they are even less precise than a full-size mouse.

It would be nice to have:

• Sliders that scale with the size of the window, i.e. the user can choose the tradeoff between screen real estate used and precision of mouse-use needed. This might necessitate a UI option to choose vertical vs. horizontal sliders.
• Simple input boxes that allow the user to easily input exact numbers (or c&p them). This would also solve the dB scale issue where the bottom end of the scale never needs as much precision as the top 6dB or so.

If the "twiddly knobs" UI is desired, make the two alternatives an option that can be switched in a Preferences window or similar. It probably would also be a nice option to have for larger devices where a bazillion sliders are just not feasible to use on limited screen sizes like laptops.