Update Appdata and DOAP
This updates some URLs in Gajim’s appdata, desktop, and DOAP files.
This also unifies Gajim’s subtitle to: 'A fully-featured XMPP chat client'
Note that I intentionally added 'chat' here, in order to increase the visibility of Gajim (for example Flathub’s search engine uses the title + subtitle exclusively for search terms).
@andre I hope the changes I’ve made to Gajim’s appdata.xml.in are ok?