GUI: try to detect Gnome font preferences as well and cache the result

Thanks to Caspar Schutijser (cschutijser on IRC) for reporting a regression
in commit cff3b86 on his system.
GUI: enable message scrolling via Home and End

Even with a pretty animation.

Signed-off-by: Jan Kundrát <jkt@flaska.net>
GUI: remove whitespace from the output of gconftool-2
GUI: try to detect Xfce and then use the Gnome font preferences
Submission: don't use BURL when the IMAP server doesn't support it, but still enable CATENATE when it's there

Thanks to Sebastian Spaeth <Sebastian@sspaeth.de> for reporting.

fixes #636
GUI: Add some short text on what BURL actually is

fixes #633
GUI: place focus on the first sender when composing a new mail and to the body when replying

Thanks to Sebastian Spaeth <Sebastian@sspaeth.de> for reporting.

fixes #635
GUI: remove ambiguous Ctrl+N shortcut

This was introduced by me in commit:5ec815a3e15ba5939326f77f30024671232fb4eb.

Thanks to Sebastian Spaeth <Sebastian@sspaeth.de> for reporting.

fixes #631
Destroy the models immediately when returning from the settings dialog

Previsouly, there was a possibility that the SQL cache might be involved in some
activity at the time the nukeModels() was called. However, due to the delayed
deleting, the Model's actual destructor (which destroyes the cache, which ends
the active transaction) might be run only after the setupModels() has finished,
which meant that the newly-created sqlite connection hit a locked database.

Thanks to Stefan Kögl <stefan.koegl@tu-dortmund.de> (hotte on IRC) for reporting.
Be nice to the IMAP server and logout when changing settings
sql: unregister the DB from the Qt's registry as well

There's still one more warning about the connection being in use. I obviously
dislike this reinventing of RAII in here (or rather in QSqlDatabase, because one
apparently cannot do it without going through its global registry, yuck!).
Fix typo: servier->server
rfc2047: treat raw data as data in UTF-8

This is in a blind gamble that utf-8 is going to be more popular than latin1.
There's no right thing to do.
rfc2047: treat unrecognized encoding as utf-8 and don't enumerate the list of known codecs
Add support for decoding of RFC 2231 parameter names

refs #639
refactoring
Add support for encoding of RFC2231-styled parameters

refs #639
Composer: encode the file names using RFC 2231

fixes #639
hide/show message pane
travis: keep up with changed repository names

The joy of relying on 3rd-party repositories -- behold!