When a tab has multiple split panes, updateAggregatedTabStatus picked
the highest-priority status across all panes, with the focused pane only
breaking ties. A background pane with a higher-priority status (for
example an idle pane when idle outranks working in the user's priority
list) would override the working pane the user was looking at.

Now the focused pane (self.activeSession) drives the subtitle whenever
it has a status, falling back to the highest-priority sibling pane only
when the focused pane has none.