GenericUI: rationalize Autostate button sizing

GenericUI: rationalize Autostate button sizing

When there was a knobtable in the ControlUI, the behavior of the
automation state button changed by displaying one-char state as in track
headers. Factor out that logic to depend on a new boolean property of
the ControlUI, which should be set with set_short_autostate().

This regroups sizing logic in a single place and avoids future
discrepancies between the actual caption and the size allocated for it,
when several use-cases for smaller buttons are added.