setStringValue: was dropping the subtitle whenever the current style's
supportsMultiLineLabels was NO, merging "title subtitle" into stringValue
and leaving subtitleString nil. If the style or bar height later changed
such that multi-line was supported, the cell still had no subtitle to
draw.

Always split title/subtitle into the two ivars and let Yosemite-style
recompose them inline at draw time when its bar is too short for a
separate subtitle line. Tahoe and Minimal at their normal heights draw
them stacked as before.

Surfaced by the new "Show status in tab subtitle" preference when
switching from Compact to Minimal mid-session, but the same stale-state
issue applied to any caller of setStringValue: with a multi-line label
under a single-line style.