Sync the branch graph on project saving

* Since 8642393 the branch graph is not necessarily rebuilt on project loading. It may however be out of sync at the time of project writing, if e.g. points have been deleted with no branch graph sync after that

* In order to achieve a consistent situation the branch graph is synced before saving. It is actually only synced, if it was found to be outdated.

src/main/java/org/mastodon/mamut/io/ProjectSaver.java

@@ -261,7 +261,10 @@ public static synchronized void saveProject( final File saveTo, final ProjectMod
 		try (final MamutProject.ProjectWriter writer = project.openForWriting())
 		{
 			MamutProjectIO.save( project, writer );
+			// Synchronize branch graph with main graph before saving. This is required to make saved state consistent.
+			appModel.getBranchGraphSync().sync();
 			final Model model = appModel.getModel();
+			// Save Raw Graph Model
 			final GraphToFileIdMap< Spot, Link > idmap = model.saveRaw( writer );
 			// Serialize feature model.
 			MamutRawFeatureModelIO.serialize( appModel.getContext(), model, idmap, writer );