Make sure QApplication is destroyed on exit() call

The previous code introduced at

    fe334292 "Fix crash when KDBusService exits"

hooked up into `atexit()` call. This introduced some non-obvious
dependency which later started causing double-free since commit:

    d5d8496c "Use RAII to manage QApplication memory"

It turns out, there's easier way to make sure a destructor is called by
`exit()` function: simply make it static.

So this commit reverts both fe334292 and d5d8496c, and then simply
declares `app` variable as `static` + adds an informational comment

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>