Use Shell.setModified() in SaveHandler to set the modified state of Eclipse shell on Mac.
Shell.setModified() doesn't do anything on other platforms. This was working with
SaveAction, but not with SaveHandler.

Change-Id: I05458282d1d1dad0c7aa1202d99c4c6dcddef042
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/179752
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Lakshmi P Shanmugam <lshanmug@in.ibm.com>