KDE QFileDialog helper: support name filters without parenthesis.

Summary:
The QFileDialog documentation says these are equivalent:
  dialog.setNameFilter("All C++ files (*.cpp *.cc *.C *.cxx *.c++)");
  dialog.setNameFilter("*.cpp *.cc *.C *.cxx *.c++");
so we should support both syntaxes.

Test Plan: See autotest addition; also tested together with my kdialog port to QFileDialog.

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6351