Remove dependency to KDE extragear (KTorrent).
Just create an svn external to the lib we need (libbtcore).

For now, the svn external points to KTorrent trunk. As soon as KTorrent 3.1 has been branched, we will switch over to that.

It is possible that things could break for a short time if massive changes happen in KTorrent as long as we stay with KTorrent trunk.

Also do not link against libbtcore, but compile it into the plugin. This way we can prevent problems people had in the past when LD_LIBRARY_PATH has not been set.