gwenhywfar: Support Gtk 2, Gtk 3 and Qt 5 GUIs

So far we used only the defaults, which are gtk2 and qt5. However
nowadays most applications have already switched to Qt 5 and Gtk 3, so
let's aim for that instead.

The reason Gtk 2 is still there is because GnuCash in nixpkgs is using
the Gtk 2 interface, so we can drop it as soon as a newer GnuCash has
landed in nixpkgs.

Right now the only package that depends on this is GnuCash (and
aqbanking of course), so I've tested this by not only building
gwenhywfar and aqbanking but also GnuCash. I did however not test
whether HBCI functionality works, only whether GnuCash starts up.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @goibhniu, @peti, @domenkozar