Don't truncate filter name containing parenthesis

When extracting the part of the string that contains the
filter name, use the part in front of the *last* parenthesis,
not the first one, since otherwise the name gets truncated
if it contains parenthesis itself, e.g. in a call like

    QFileDialog dialog;
    dialog.setNameFilters({"ODF Spreadsheet (.ods) (*.ods)"});

This is in line with the fact that
`QPlatformFileDialogHelper::cleanFilterList` just considers
those extensions in the last set of parenthesis and aligns
the behavior with the non-native QFileDialog implementation
that the Qt library itself provides, as well as the KF 5
implementation.

BUG: 483350
(cherry picked from commit 448b8aef)