GNOME Games should have a mobile-friendly layout

The current UI has a number of problems:

• Collection view has a sidebar. It should probably be put into a HdyLeaflet, but it's unclear to me how it exactly it should behave; !281 (merged)
• Collection view also has a stack switcher in headerbar. It could be put into a separate GtkActionBar at the bottom of the window, but it's too wide to fit on a 360px wide screen, so needs an alternate design; !281 (merged)
• Search bar in collection view is too wide for mobile and doesn't shrink. It could be put into a HdyColumn, but that doesn't work with GtkSearchBar in GTK3. It works in GTK4 though, and libhandy will soon have an alternative search bar widget that we will be able to use; — the widget has arrived, see !207 (merged)
• Preferences window has a sidebar, so should use a HdyLeaflet. An early prototype is available in a test branch; — !155 (merged)
• Right pane lists in preferences window should use HdyColumn to avoid side margins on mobile. A prototype is available in the test branch; — !155 (merged) !159 (merged)
• Gamepad and keyboard configuration has an inherently wide layout, so needs design; — @bertob said it's a non-issue
• There should be touch controls for libretro games. Needs design.
• Preferences should be a view instead of dialog on mobile. (see libhandy#52, but it's doable without it)
• Primary label on ErrorDisplay is not wrapping !281 (merged)
• Covers are too small on 360px-wide window !283 (closed)
• Search does not work well, see #189 (moved)

While it's unlikely that gamepads or keyboards will be used on a phone, the layouts still need to be adapted so that it's possible to shrink the window on desktop as well.