Add appstream metainfo.xml file.

Created by: GeraldJansen

Added an metainfo.xml file for Linux software gallery integration.

This is simply an update of PR #336.

Co-authored-by: Mathias Mailänder (@Mailaender).

WIP: I am not 100% sure this works. I tried to validate the xml with appstreamcli validate and got some warnings (eg. screenshots don't yet exist at their eventual destination in master). After installation (in /usr/share/appdata) and running sudo appstreamcli refresh --force, I still didn't find hamster or "time tracker" in Ubuntu software. Tried moving the file to /usr/share/metainfo where most apps now put the xml files (on Ubuntu at least) but no joy.