Coding style: fix header names to use the directory prefix
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.
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
test: reinitialize the DB between each test

The old init vs. initTestCase problem.
tests: Measure performance of the SQL cache when it comes to flag access

It is suprisingly slow; one second for 100k items on a memory-backed sqlite DB
on a Sandy Bridge CPU is just shameful. Perhaps the string keys are to blame
here?

(Idea shamelessly stolen from the KDEPIM guys who were recently hitting a very
same problem.)
wip: make an isolated test case for the SELECT benchmarking