[INTERNAL] Add IWorkspace and IWorkspaceRunnable to Saros/Core

These new interfaces come along with Eclipse implementations/adapters.

Change-Id: I3c1882f4f38618e094cc288f8ae1580df275526a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1564
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>