To have the name and summary translatable, rename as .in file, add
underscore on localizable tags and add to POTFILES.in.
Also clean out trailing whitespace and weird characters, add indentation
and fix tag s/updatecontact/update_contact/.
Finally add an appstream-util validation so that the file syntax is
checked during a `make check`.
This way we can be warned quickly about any AppStream issue (cf. bug
782759). This test requires web access for screenshot verification.
Packagers are invited to use --without-appdata-test option if they want
to skip the test (for instance if build environment has restricted
network access).
