update appdata to pass flathub validation

[nedrichards]

Flathub requires more tags in the appdata to provide a richer store experience. They're also used by other Linux application centers like GNOME Software and KDE Discover.

• add version tags
• add OARS information https://hughsie.github.io/oars/
• the other changes come from running

  flatpak run org.freedesktop.appstream-glib upgrade glabels-3.appdata.xml.in

to test:

flatpak install flathub org.freedesktop.appstream-glib flatpak run org.freedesktop.appstream-glib validate glabels-3.appdata.xml.in