PersistentStore: expose deinit() as a static function
PersistentStore: expose deinit() as a static function

This is needed (publicly) in the test suite.  I considered hiding this behind
an #ifdef, but we still need that function (privately) in the main app.

This way, we can call
    PersistentStore::deinit();
directly instead of
    PersistentStore::instance().deinit();
which would create a new PersistentStore if we didn't already have one.

Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui