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
travis: keep up with changed repository names

The joy of relying on 3rd-party repositories -- behold!
GUI: systray support

fixes #67
REVIEW: 110461
Coding style cleanup
Add a proper credit for Stefan
GUI: make the settings dialog in sync with the default value of the "show systray"

refs #67
CCMAIL: stefan.koegl@tu-dortmund.de
GUI: add a tooltip to the systray

It shows the number of unread messages, if any.
Rework the drag & drop in the message list view to only skip UID=0 messages

The whole stack assumed that a message can only be drag-and-droped after it has
been loaded. That's clearly nonsense, the only real requirement is that the
knowing the UID is enough.

This was rather annoying when selecting messages via Ctrl+A on a testing
instance with plenty of messages and no persistent cache (and hence 45k+
messages not loaded which could not be moved via Ctrl+A).

The new code has a nice side effect of always rendering the "loading-UID-not-known"
messages as disabled, greyed-out entities, which is also kind of cool.

Thanks to Stefan de Konink <stefan@konink.de> (skinkie on IRC) and Brian Jackson
<iggy@theiggy.com> (iggy on IRC) for their reports about slow drag-and-drop
(which is not fixed yet).
dnd: don't use magic numbers
dnd: include only a limited number of pixmaps in the drag indicator pixmap
Workaround for segfault when sending a message after the model has been replaced

The proper fix would have to be much more intrusive. Let's leave the bug open
for a while, the current experience is better than segfault, but still leaves
much to be desired.

refs #640
systray: Add a nice background and a variable font size for the unread message count
systray: simplify the icon and tooltip handling
Simplify: no need for "this->"
Inform about unread messages even when loading from cache