Changes

[518790]Change save behavior of ActivityExplorerEditor.

Upon save, instead of directly saving the Sirius Session, we rely on the
org.eclipse.sirius.ui.uiSessionFactory extension point to provide the
Saveables which provide the saving behavior. This allows products
integrating Amalgam to customize the save behavior of Sirius Sessions
from the Activity Explorer editor.

Bug: 518790
Change-Id: Ic3e5eec1d15e56f5d34f4a6f4591403b42590aca
Signed-off-by: Florent Latombe <florent.latombe@obeo.fr>