Set the toggled default metadata of parameters.

This is so generic code looking at the Parameter for gain, solo, and
rec-enable will get the correct value for toggled.
Remove redundant calls to start/end_touch.

The controllers do this automatically.
Use a button as controller for toggled parameters.
Fix crash when deleting all points from a line.
Show discrete/toggled automation as stepped line.
Somewhat working gain automation.

This probably isn't correct in several ways, but it works more than it did, so
I figure it's push worthy.

Still not working:

 * Saving mute automation list
 * Dragged control points are not snapped to model restrictions
   (boolean, in this case, but general problem)
 * Line goes funny if you record mute automation
   (as opposed to drawing it which works)