KUrlRequester::setFilter: note special filter syntax, allow unnamed filters

The API dox referred to QFileDialog::setNameFilters for the syntax to use
which is "Comment (file pattern)". The internal logic though expects
the old KDE style syntax "file pattern|Comment".

For more compatibility with Qt also allow name/comment-less filters by
handling of non-existing "|" hit in the filter parsing.

CCBUG: 464350
CCBUG: 369542
(cherry picked from commit 6e945ed1)