Revamped a lot of the settings stuff. This should fix the Q_ASSERT that would appear

when the quasselcore was quit, due to gui types stored in the config...

I took the opportunity to change the organization name to "Quassel Project", which affects
the location of the QSettings files. Also, we now use two separate files for client and core.
Old core settings and some client settings are automatically migrated. Some UI settings are lost,
however. :(

Further reduced the usage of QSettings where it doesn't belong.