Provide Quassel::configDirPath() and Quassel::findDataFilePath()

Provide Quassel::configDirPath() and Quassel::findDataFilePath()

These functions are about to obsolete their pendants in util.cpp. With KDE integration
enabled, data files will (only) be looked for in KDE's data directories rather than XDG.
This is in preparation of build system changes that will install stuff into KDE's dirs.
Still lacks some stuff to work properly on Win and Mac, I guess.

Instead of --datadir, use --configdir now. We'll move all user files in one location soon.