Add form factors to desktop and appdata files

See merge request !4

This makes the app visible in the PureOS Store on the Librem 5.

See https://puri.sm/posts/specify-form-factors-in-your-librem-5-apps/

This improves the internationalization, because each language
has a different pangram, to see the font applied in all the
alphabet characters.

Fixing

See merge request !1

style: Add minor styling tweaks to the menu for mobile