Add QUrl based static methods to QFileDialog

With QUrl variants of the static methods, it is possible to get to use
VFS facilities of the platform if available.

Since we can't predict if the application will use the VFS available in
the platform or its own mechanisms, an extra parameter is provided to
restrict the protocols allowed to the user. This extra parameter
defaults to no restriction, which is the most convenient if the platform
file dialog and the application use a matching VFS. It's likely to be
the most common use.

Change-Id: I4c9effde9d194d226cd8b7a140eb9036187ba87b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>