common: Make the Quassel class a proper singleton, clean up

common: Make the Quassel class a proper singleton, clean up

No longer have the various application classes inherit from the
Quassel class, which is a precondition for making the latter a
QObject down the line. Enable application-specific config reload
and quit behaviors by providing the means to register handler
functions, replacing the previous inheritance-based mechanism.

Make the Quassel class a proper singleton, remove inline functions
and modernize the code.