update contact instance in chat control when a resource comes back online (a new contact instance is created). fixes #2619

don't show contact in roster if account is offline and no pending events and show_offline is False. fixes #2618

again fix Gajim so it runs if NM is not there. Stefan please have a look from time to time in the timeline to see other ppl fixes [f.e. this was done by Yann b4]

(options are in ACE only)

missing logic on prefs window for exo-open stuff. fixes #2606. thanks baze for ur attention to detail

mention the link to use more music track players in the tooltip of set_status_msg_from_current_music_track_checkbutton

some events are not readable. thus unread event does not make sense. Pending Events is what we wanna say

if user is xfce4 user [startxfce4 or xfce4-session in running procs] then use exo-open (needs libexo) to open urls using the xfce defined default apps

(should find a better place) - see #860