First step to introduce dynamic roles

Currently Dolphin is only capable of showing a fixed number of
roles (e.g. name, size, date, owner, ...). Dolphin 2.1 should
support also other roles like rating, tags, comments or other
meta-information of images or audio-files.

To support this the sorting-type and additional-type from
DolphinView has been replaced by a role-type represented as
QByteArray.