Handle the roster order cache as a real cache

When an external (or internal) event may cause the order of the
cache to be modified, or new elements to be added, schedule it
for a rebuild. Otherwise, don’t, and only rebuild it when
refreshing (that should improve refresh speed a lot).

Also, if the position in the roster is further than the total size
of the roster, go back to the top instead of displaying an empty
window with “+++”.

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