Rewrite command line option parsing
Use QCommandLineParser for that. A couple of behavior changes:
* Profile names starting with a '-' are now allowed.
* Specifying an URL that doesn't start with "mailto:" is now an
error.
* The feature to activate debug traffic logging to disk may now be
enabled by specifying -l in addition to --log-to-disk.
* Specifying -p multiple times is not an error anymore; the value
specified last is used.
As a result of using a real command line argument parser, we may now
combine arguments when invoking trojita, e.g. trojita -lp work.
While there, slightly reword some text.
Change-Id: Ic97470837b530ed60e54f513eb4b834a3a6a4f39