KCoreDirLister::nameFilter: revert to non-partial matching again

Commit 192b2baf added the un-anchoring
of the regex pattern in setNameFilter().

Having unanchored matches though breaks some expectations from wildcard
expressions as used with name filters (see also examples in the API docs
of setNameFilter).
E.g. for "*.po" one would only expect files with suffix "po" only, not
also "pot" or any other suffixes starting with "po".
Similar "*.tar" should not deliver "tar.gz" files etc.

The olc QRegExp code also was just using
QRegExp(filter, Qt::CaseInsensitive, QRegExp::Wildcard)
so also not doing partial matches of any kind.

(cherry picked from commit 6b58574d)