Added automatic generation of lookup_int callbacks for collections, for
quicker lookup by index instead of looping over the whole thing. Import
is still quite slow, though now it only takes a few seconds.
The next bottleneck seems to be running update (depsgraph, notifiers, ..)
on setting every property. I fixed part of that by avoiding a notifier
to be added each time, now it checks for duplicates.
64078786cc