metainfo: Add device-specific metadata

[1peter10]

Thank you, @tchx84, for creating this very helpful app!

This PR will (after a subsequent release) make Portfolio show up in Flathub's Mobile Collection: https://flathub.org/en/apps/collection/mobile/1

See https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/\#mobile-only-apps for information regarding the change.

Mobile only apps

Mobile and tablet only apps typically should have:

<requires>
  <control>touch</control>
  <display_length compare="le">1279</display_length>
</requires>

Desktop and mobile apps

Apps that support and work on mobile, tablet and desktop typically should have:

<recommends>
  <control>keyboard</control>
  <control>pointing</control>
  <control>touch</control>
</recommends>
<requires>
  <display_length compare="ge">360</display_length>
</requires>

I chose mobile-only as

<value key="Purism::form_factor">mobile</value>

suggests that this is most fitting.

[tchx84]

hey @1peter10 , thanks for looking into this, just left a minor comment.