After a9f3a3d saveSettingsPrivate() errors out if the config file
doesn't exist. This is because QFile::open(QIODevice::ReadOnly... )
doesn't create a new file for non-existant filenames, like the
same invocation with the previously used QIODevice::WriteOnly does.
I'm not sure how likely the absence of the config file is, but it's
easy to recover from by creating an empty file first. We're trying
to save our settings afterwards anyway.
A place there this problem occurred is the configsave unit test. The
filenames passed in ConfigSaveTest::testGtk2 don't exist when running
the test, causing it to fail without this patch.