Coding style: fix header names to use the directory prefix
Composer icon sketch

Attached are svg's and PNGs in ~target size, a small and a large variant.
I just plundered the oxygen kword icon for this and rounded the capped "3"
GUI: use different icons for different window roles

I'm experimenting with a vertical DE panel which means that I only get an icon
for each window which makes it pretty hard to distinguish among them. Let's use
different icons for the main application window, for an individual message, for
the composer and for the headers/source view.

v2: Use Thomas' icons for the composer
GUI: show our own icon when displaying attached messages

See the comment within the code for motivation behind this.
GUI: show message subject in the composer's window title
Merge branch 'gui-icons-and-fixes'

REVIEW: 114171