Fix performance regression when loading folders in Details View

When using Details View, only insert all pending items immediately if
new items are inserted which might be children of a pending item. Fixes
the problem that inserting the items in multiple bunches slows down the
folder loading.

Note that the cause of the slowness when inserting in multiple bunches
is that KItemListSizeHintResolver needs O(N^2) time in the worst case
for inserting N items into a model with N existing ones.

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