common: Rework command line option handling

Since we can now rely on Qt5's QCommandLineParser, there is
no longer a need for an abstraction for command line parsing.
Use QCommandLineParser directly, and remove the now obsolete
AbstractCliParser and its various implementations.

Move command line parsing into Quassel::init(), which allows for
using translated strings. Make user-visible strings translateable.

Remove the long-deprecated --datadir option for now (may come
back if we decide to allow for specifying the database location
independently of config files).