Move statistics from config to their own place
Statistics data is not saved to config file anymore, instead it has its own
file, statistics.dat.
Advantages:
- config file is not being overwritten continuously,
- statistics are saved only when necessary,
- config is only saved when changed. [*]
Backwards compatibility:
- Downgrading to an older client: the old client will see as if the statistics
has been reset, and thus starts counting from 0.
- Upgrading to a new client: the new client will correctly see the progress
the old client made, and sum it up with the statistics it already might have.
[*]: Actually, this is not really true. There's no real change required to make
the config be saved, just pressing "Ok" in the Preferences will do it. And this
still only applies to the daemon, because monolithic and remote gui will save
their GUI-related settings (position, etc.) unconditionally at exit.