Changes

[Bug 522623] Fix 'save all' conflict with Sirius Editor
Cherry-pick the 3 commits [1], [2] and [3] from branch 1.9.x to this 
branch:
* Makes ActivityExplorerEditor implements ISaveableSource in order to 
avoid parallel save when a Sirius Editor is open and using 'save all' 
action.
* Remove doSave() implementation.
* Fix regression: Since ActivityExplorerEditor became a 
ISaveablesSource, closing Eclipse with a dirty session does not propose
to save changes because ActivityExplorerEditor.isSaveOnCloseNeeded()
method return false.
[1] 
https://git.eclipse.org/r/plugins/gitiles/amalgam/org.eclipse.amalgam/+/6128bf3114a006c059a47ab81fa92318d36abcc4
[2] 
https://git.eclipse.org/r/plugins/gitiles/amalgam/org.eclipse.amalgam/+/a0fb6293a96eb415d10ceee768db97f04124032a
[3] 
https://git.eclipse.org/r/plugins/gitiles/amalgam/org.eclipse.amalgam/+/4d2409dedf26f001b6c76553b6da359c933a522d
Bug: 522623 Change-Id: I300937a9f9eae273b17d6be409915c38c9c5e3ff 
Signed-off-by: Laurent Redor <laurent.redor@obeo.fr>