FileChooserDialog constructors taking backend argument

[Daniel Boles <dboles src gmail com>]

Hello,

About this commit: https://git.gnome.org/browse/gtkmm/commit/?id=bd97e557771cdce23bdc815288db2ffdd8c083c9

Clearly these constructors could not actually achieve their stated purpose in GTK+ 3, as the backend functionality was removed already then.

In case anyone was trying to use these in gtkmm-3, should deprecation warnings be added there, too? Clearly we don't want to just remove these ctors and break code that was using them, however ineffectively.