Set file dialog default directory

[aspschn]

When I click 'Open folder' button, the default directory of file dialog is the same as the executable is located.
However, if this program is intended to be used on Linux, it would be more convenient to point to the usual cursor theme directory such as /usr/share/icons.
This could be achieved with code like this:

QString path = QFileDialog::getExistingDirectory(this, "",
    "/usr/share/icons");

If this program is developed to be cross-platform, this code may cause errors. So it might be a good idea to check if a directory exists before.