Consider adding libhandy module

GNOME runtime will stop supporting the libhandy and GCR-3 modules with version 51.

Some apps rely on it, and it would be good to support it via the shared modules.

More information: https://discourse.gnome.org/t/upcoming-gnome-51-flatpak-runtime-will-drop-gcr-3-and-libhandy/35111

For example:

• Consider using libhandy from runtime io.github.helpseeker.Gyre#8
• https://github.com/flathub/com.doycho.euterpe.gtk/blob/master/com.doycho.euterpe.gtk.json#L32
• Use libhandy and libgee modules from runtime com.github.birros.WebArchives#8
• https://github.com/flathub/org.gnome.Lollypop
• https://github.com/flathub/com.github.childishgiant.mixer/blob/master/com.github.childishgiant.mixer.yml#L38

And many GTK 3 applications.