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Appstream nowadays prefers `.metainfo.xml`:
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html

appstream-util version 0.7.16 complains:

  $ appstream-util validate-relax --nonet ./data/sm.puri.Calls.appdata.xml
  ./data/sm.puri.Calls.appdata.xml: FAILED:
  • tag-missing           : <description> required
  Validation of files failed

What it actually means that the <description> is not well formed because
it should contain a paragraph.

Closes #73

This adds an appdata, installs it and adds a test to validate it.