A fairly simple feature - preferences for velocity sensitivity, but a
fair amount of UI infrastructure. This patch includes a new
KnobPreference, changes to KnobView to support a horizontal layout
(which works better in a layout with more compressed vertical space),
and of course the plumbing of the preference itself.
Also some rework of the touch handler in the KeyboardView. This patch
simplifies the logic a bit and fixes a long-standing bug in which the
pressure was always read from pointer index 0 rather than the pointer
index of the key being pressed.
Raph Levien