Swipeable Leaflet

Since there's no proper issue for this yet I'm opening this to track the design and current progress. The basic idea is that we want all Leaflets (and eventually sub-page navigation in stacks, too #28 (closed)) to have a swipe gesture that can replace the back button in the top left.

As far as I'm aware, the main missing pieces currently are:

• fix and merge over/under mode transitions !230 (merged)
• review and merge HdySwipeable !309 (merged)
• fix and merge dim bottom child during transition !226 (merged)
• merge unify leaflet transitions !326 (merged)
• Implement back/forward gesture for HdyLeaflet !332 (merged)

This blog post provides some more background (though exalm said there's been progress on the shadows part since then) https://blogs.gnome.org/alexm/2019/09/15/gnome-and-gestures-part-2-hdyleaflet/

This would fix #10 (closed)