[430724] Only enable the new saving policy if XText resources are present

Instead of enabling the new saving policy for all sessions as soon as
the Sirius/Xtext bridge plug-in is installed, enable it only when at
least one Xtext resource is added. We do not revert to the default one
if all Xtext resources are removed, but this should not happen very
often.

Note that with this change, even the Xtext-specific saving options are
now only enabled if Xtext resources are present, not just the new saving
policy.

Bug: 430724
Change-Id: Ie13112a6468243e6bce24beab1c32026dcc969a5
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[test] Fix wrong tooltip text detected by SWTBot test

The test TabBarTest fails randomly. The tooltip of the "Activate
Layouting Mode" button is sometimes setted with "&Zoom". This tooltip
corresponds to
org.eclipse.gmf.runtime.diagram.ui.l10n.DiagramUIMessages.ZoomActionMenu_ZoomLabel
that is used in ZoomExtensionContributionFactory.
The code in ZoomExtensionContributionFactory is not reliable. Indeed, we
set the tooltip of the last item of the tabbar and sometimes, this is
not the right one.
We now use the "index" parameter.

Change-Id: I672ef8db3ca03987f72f903160d23e307d7d4dbb
Signed-off-by: Laurent Redor <laurent.redor@obeo.fr>