qt4-b-gone: Reorganize the initialization sequence

KDE4 required special handling during initialization, because command
line arguments needed to be parsed before instantiating the
application object. Since this is no longer required, the sequence
can now be simplified.

To prepare for enabling early translations (also for the CLI parser),
instantiate the application first, then explicitly initialize the
Quassel instance in main() (which will later both load translations,
and handle CLI options). Only then also initialize the application
itself.

Since initialization errors are handled by exceptions anyway, get rid
of the return values for the various init() methods, too.