[FIX] Ignore already shared files while adding new ones to a session

When adding files (or more general, resources) to a partially shared
project, used to exclude already shared files from the selection.
That mechanism broke when the list of already shared resources was
changed from Eclipse resources to Saros Core resources (whereas the list
of selected resources remained Eclipse-ish).
This patch restores the old logic.

However, higher-level logic seemed to cover that defect, so that no
failures were observable.

Change-Id: Id26705e394245de86fa45a15e0c9cd7090096c74
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1498
Tested-by: Jenkins CI
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Andrej Szaffranietz <a.szaffranietz@gmail.com>
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>