fractal-gtk: Use HdyLeaflet to adapt to small sizes

Fractal's main view could only shrink so far, barring usage at smaller sizes such as a phone's screen. HdyLeaflet enables Fractal to adapt.

Related to https://gitlab.gnome.org/GNOME/fractal/issues/377

The only issue is https://source.puri.sm/Librem5/libhandy/issues/63