[AppStream] Information about "mobile-readiness" in AppStream

[Matthias Klumpp]

Hi!

One of the most requested features in AppStream over the past few
years was how to get information about "ready for a mobile device"
into the specification, so a device running Linux on mobile could
filter the catalog of applications to select only the ones where the
application author has marked them as "supported on mobile devices".
Now that we at Purism are building a Linux phone, the issue has
surfaced again, and I think it's time to finally solve it.

One of the big issues in past discussion was to define what a "mobile
device" even is. My a attempt at a definition is the following: A
mobile device is any computer device that is usually carried around by
a person and operated using finger/touch input".
I was made aware at FOSDEM this year that apparently also some apps
have screen size lower limits, so for me at the moment it makes sense
to define "mobile readiness" as the result of a minimum screen size
and the ability to be controlled by touch.

In previous discussions (at
https://github.com/ximion/appstream/pull/192), a "supports" tag was
suggested to indicate certain hardware that the application explicitly
supports and works well on.

Using that, we could have something like:
```
<supports>
  <control>touch</control>
  <control>pointer</control>
  <screen_size compare="ge" unit="in">5</screen_size>
</supports>
```

The new tags found in "supports" could also show up in a "requires"
element to set an absolute minimum screen size, or require
cursor/touch input. Apps which do that wouldn't run at all on a device
not meeting the requirements.

In Android metadata
(https://developer.android.com/guide/practices/screens-distribution),
screens are classified into "small", "medium", "large" and "xlarge"
instead iof giving actual dimensions. But Android also has the ability
to upscale smaller apps and all its apps are designed for mobile
anyway, while most of the apps we have AppStream metainfo files for
are not designed to work on mobile. Yet, doing the same here may also
be viable, if we define what a "small" screen actually means for the
app (e.g. can it be smart-watch sized?).

We could of course also only have the information about controls, as
requested in https://github.com/ximion/appstream/issues/55, and simply
assume everything that supports "touch" is mobile-ready, as you
wouldn't use touch input on your massive television screen (but apps
supporting that usecase can be filtered out via a "tv-remote"
control).

There is also the question of whether we actually need a "supports"
tag, and whether we shouldn't simple reuse the "requires" and
"recommends" relation tags already found in AppStream and just permit
recommendations about controls and screen sizes.

This mail is primarily intended to start a conversation, and I will be
very happy about any possible feedback, so we can implement this
feature the right way(tm) from the start.

Cheers,
    Matthias

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