[INTERNAL] SPathDataObject no longer serializes IPath objects

Instead just the plain string is send. This is necessary because
serializing concrete IPath implementations would break support for
multi-platform support eg. Eclipse <-> IntelliJ session.

Change-Id: Ieb8f864d0c15a6196a4d95f6421a492193dc42fb
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1380
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>