Finish the file dialog tester applet

This allows us to exercise the KoFileDialog abstraction in all
permutations. Use this to check that:

* the filename is returned with the correct extension
* the last visited directory is used when opening the file dialog
* the correct filter is set

It needs to be run on Windows, OSX, Gnome and KDE. On Windows, we use
the static methods to get the native file dialogs, on OSX,  we use the
non-static QFileDialog methods to get the native file dialogs.

On X11, we make sure we get the (superior) KDE dialogs when running in
KDE, but elsewhere, we use the native Qt dialog, not the GTK dialog.

CCMAIL:calligra-devel@kde.org