Add support for Unrar

I´d like to see support for Unrar´ing files in Qt (like the private QZip stuff we have - which should be made public IMO). There is plenty of free unrar implementations to pick from which we can add to src/3dparty. The reason for this I recently came across a project that needed both unrar and unzip support, and it´s slightly annoying that there isn´t any official support for this in Qt even though the zip stuff is already in place.