Design: Organize the shortcuts dialog

[sevonj]

The number of shortcuts has outgrown the current design. Organize entries using pages.

[sevonj]

Oh yeah, I forgot. I was going to do this last year, but I'm waiting for the next Ubuntu LTS for AdwShortcutsDialog.

https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/class.ShortcutsDialog.html