Use libhandy 1.2 to make UI responsive

Hey, I've found this neat application and I wanted to be able to use it on the Pinephone. So I've implemented some small adjustments to fit the UI on a small form factor with HdyLeaflet essentially. The CI tests are still failing at the moment but I think this could rely on the gtk_init() I've added to make sure hdy_init() works properly.

I'm also not fully happy with the GtkFlowBox I had to use instead of the GtkButtonBox to make it fit into smaller screen widths but it is usable on my Pinephone now and the one back button works properly in each case. ^^'