[Meta] Use standard libadwaita layouts and components

[bragefuglseth]

• Adw.ToolbarView
  Using an Adw.ToolbarView instead of a vertial Gtk.Box gives the window a flat header bar and puts its styling in line with other modern GNOME apps.

• GTKShortcut
  GTKShortcut should be used to show keyboard shortcuts https://developer-old.gnome.org/gtk4/stable/GtkShortcutsWindow.html

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Renamed the issue to track more changes to bring the UI more in line with other libadwaita app. Putting it together as my first libadwaita/gtk app, there are a lot of non-standard layouts, including f.e the keyboard shortcut menu.

Feel free to list more below and I will add them to the list.

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