[API] Amend core filesystem interfaces

Under the headline of decoupling Saros from Eclipse, this patch amends
the Saros Core filesystem API by adding all methods that are actually
called on the instances of the Eclipse interfaces (objects of type
IResource, all its sub-interfaces, and IPath).

Please note that these interfaces are under development. They will be
simplified and will finally provide less methods with less parameters.
After all, they should hide IDE-specifics. For now, that simplification
is deferred until the IntelliJ adapters are written.

Change-Id: I7e25d5cf15f01222c174e393a4885c50d2913e16
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1478
Tested-by: Jenkins CI
Reviewed-by: Stephan Salinger <stephan808@googlemail.com>
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>