Show all items in the directory when the name filter is cleared

The problem was the following: While a QMutableSetIterator iterates the
items in the QSet m_filteredItems, all items that match the new filter
are removed from the set. However, to ensure that the iterator still
reaches all items in the set, one must not use QSet::remove(), but
rather the iterator's remove() method (see QMutableSetIterator docs).

CCBUG: 300504
(cherry picked from commit 288473a9)