Currently, we just skipped the notification comparison, if the
notification had a raw icon attached. This is a bit counterintuitive.
Calculating a checksum of the raw icon's data is the solution.
For that we cache the pixel buffer and introduce a field, which saves
the current icon's id. The icon_id may be a path or a hash.
So you can compare two notifications by their icon_id field regardless
of their icon type by their icon_id field.
While running under valgrind, it may take over 30 cycles in the loop to
wait for a running GTestDBus instance. So we have to increase the
waiting time.
Waiting a whole second will change the signal/noise ratio and will only
fail, if there's really a problem with the GTestDBus usage.
And as we're sleeping 500us in every loop, we usually get a super fast
response, too.
