stable repofilter: Remember state (shown/hidden) of the repo filter bar

The state (shown/hidden) of repo filter bar will be remembered _per repo_.
This seems reasonable because each repo can have its own repo filter bar with
its own settings and its own state. So saving the state per repo avoids
collisions when all repo tabs try to save their own state when the workbench
is closed.

On the other hand, this is contraty to what is done for other repo filter bar
settings. Actually, it seems that collissions in these other settings may be
more likely than for the filter bar visibility. Perhaps some of these other
settings should be saved per repo as well?