Revise some text strings

This is an attempt of revising some text strings to achieve better consistency (fixes #9548 (closed)).

Modifications done so far:

• Replace autojoin by Join Automatically
• Replace Disconnect in group chat menu and roster by Leave
• Address contacts without using he/she/other
• Rewording of dialog_messages.py
• Rewording for settings in config.py
• Replace Audio/Video Session by Voice/Video chat
• Replace Cert by Certificate
• Replace HTTP Upload by HTTP File Upload to use the correct term (which may be hidden anyways in later versions, this is too technical)
• Replace Nick by Nickname
• Replace MUC, Conference and Room by Group chat (see docs.modernxmpp.org)
• Replace Groupchat by Group chat (see docs.modernxmpp.org)
• Replace Occupant by Participant (see docs.modernxmpp.org)
• Replace Roster by Contact list (see docs.modernxmpp.org)
• Replace Jabber-ID, JabberID and JID by XMPP Address (see docs.modernxmpp.org)
• Replace Jabber by XMPP
• Replace GTK+ by GTK (rationale).