Stretch: overhaul slider aesthetics

[broadwell]

Some specs:

• Must be able to display the label above, including the keyboard button mappings, if any
• Must display the current value in a consistent location
• Should be able to add ticks (not necessary by default, though) at specified intervals, ideally with the max and min values and some interim values labeled. This would be useful for the transposition slider, and perhaps others.

One path for aesthetic improvement would be to make them look more like the actual sliders on player pianos (some examples are in the project docs; other glimpses here and here), but actual skeuomorphic resemblance probably isn't necessary.
I happen to think a skinny rectangular slider knob might be preferable to the current circular widget, but that's just a personal preference.