Window managers are such a crap. So on top of all friendly hinting and
geometry setting, simply use gtk_window_move() whenever a session
managed GtkDialog is shown. It should be very hard to ignore that, but
of course the WM has the last word no matter how hard we try.

Note: this code should be more complex and the signal connection
should happen in GimpDialogFactory. Left it in GimpSessionInfo for now
until it got a bit of testing.