Engine-XMPP: contacts were able to modify local roster through nickname changes

nicknames are now only processed if there is a non-empty display name for the contact in the roster that differs from the jid
Engine-XMPP: regression caused personmodel of personchat not to be updated
Merge pull request #163 from oli-obk/fix/xmpp_nickname_overwriting
Engine-JabbR: fixed missing full stop in message

Thanks goes to Martin Bagge for noticing this.
Frontend-GNOME: fixed invisible engine assistant on OS X and Windows
Merge branch 'stable'
[Engine-VBulletinChatbox] Initial vBulletin Chatbox support.

Initial protocol module for the vBulletin Chatbox plugin.
[Engine-VBulletinChatbox] Fix skipping old messages.
[Engine-VBulletinChatbox] Leave message parsing to MessageBuilder.
[Engine] Use HtmlAgilityPack to parse HTML and improve image parsing.
[Engine-VBulletinChatbox] Implement /me support.
[Engine-VBulletinChatbox] Trace calls.
[Engine-VBulletinChatbox] Update token if sending fails; Unicode

Update the security token and, failing that, try to log in anew if
sending a message fails the first time. Include commands to do this
manually.

Also, add Unicode support (decimal XML character escapes).
[Engine-VBulletinChatbox] Fix /relogin and /retoken.

* /retoken shouldn't request the security token twice.
* /relogin should request a new security token after login.
[Engine-VBulletinChatbox] Highlight my own nickname.

A bit weird that this is done completely manually in each protocol.
[Engine-VBulletinChatbox] Improve status messages.

Now output with every relogin or retoken operation, with less copy-paste!
[Engine-VBulletinChatbox] Don't prepend event prefix to commands header.
[Engine-VBulletinChatbox] Remove outdated code.

It unwittingly survived a restructuring...
[Engine-VBulletinChatbox] Fetch security token from FAQ page.

The chatbox archive is a notoriously slow page. The FAQ page seems to be
the most efficient page in terms of processing power, although the data
itself is extracted from the global search form.
[Engine] Handle the "embed" HTML tag gracefully.