Add usage logging to the buginfo dialog

We can now log important events to a krita.log file. It is completely
independent of qDebug, and only meant for those actions that we know
users often get wrong or have problems with. Right now, it logs
creating, loading, exporting and saving a file, and switching between
the smoothing options.

The krita.log file is NOT automatically uploaded or shared or anything
like that. The header that explains this is translated so users can
see that for themselves.

We rotate 10 sessions in the log file, so even if the user starts Krita
a few times after having a problem, we should still be able to review
the problematical session.

The logging api is pretty simple: a static KisUsageLogger::log method
that writes a timestamp, and a KisUsageLogger::write method that writes
an informational line without a timestamp.

Note that the logger is created and torn down in the main function:
this means that the location is GenericDataLocation, not AppDataLocation.

CCMAIL:kimageshop@kde.org