altosui/telegps: Reduce CPU time needed for flight displays

altosui/telegps: Reduce CPU time needed for flight displays

Don't update displays which aren't shown; track hierarchy changes to
trigger display from most recent state data.

Don't update values which haven't changed; remember previous values
and compare with new before updating widget contents.

Signed-off-by: Keith Packard <keithp@keithp.com>