Add metainfo file for Linux software stores

[rugk]

The app is not shown in GNOME Software. So you may add some meta data to see it there.

See https://bugzilla.redhat.com/show_bug.cgi?id=1534140 for Fedora bug report.

[szszszsz]

Hi!
Is this the same as #274 ?

[rugk]

Hu, no?

I just want to see the app in GNOME Software, that's the "application store" in GNOME and Ubuntu e.g.
I also found out the correct term is: metainfo` file. So you need to add a small metainfo file with some description, images and so on, and then one appears un GNOME Software (and as it seems, other stores). That whole thing seems to be called AppStream:

• https://en.wikipedia.org/wiki/AppStream
• https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html

[szszszsz]

I see. Which GNOME version do you use? I remember it was working with 3.26 - the App was listed and could be executed.

[rugk]

Gnome 3.26, it is only available from Snap, but I'd prefer the official distro repo here (where it is available, see Fedora issue above). Fedora 27

[szszszsz]

I understand now. OK! I will create the metafile. It should be available from the next release.

[szszszsz]

Merged to master: 71e134f
It should show up on package update (that is, after v1.3 will be released). If not, please ask the package's maintainer. Appstream file is placed in /data and it is copied on CMake install to /usr/local/share/metainfo (or similar) directory.