Branch features disappear on project opening

[stefanhahmann]

This Pull Request changes behavior of the BranchGraphSynchronizer. The BranchGraphSynchronizer is set to optimistically assume that the branch graph is up-to-date = true at the time the synchronizer is created. The sync() method evaluates the up-to-date flag and only performs the rebuilt, if the up-to-date flag is set to false, which is usually the case, after the notifiers have informed the synchronizer that the graph had been changed.

By this change, it can be achieved that, the branch features are not reset/lost after saving and re-opening a project or by just opening a track scheme branch view.

This Pull Request adds some unit tests to test the indented behavior:

• testBranchFeaturesAfterSaveAndReload in ProjectLoaderTest tests, if previously computed branch feature still exist, if the project has only been saved and reloaded
• testBranchFeaturesAfterOpeningBranchView in MamutBranchViewTrackSchemeTest tests, if previously computed branch features still exist, after a TrackSchemeBranchView window was opened
• testKeepBranchFeaturesAfterSyncWithoutChanges in BranchGraphSynchronizerTest tests, if previously computed branch features still exist, after the sync() method of BranchGraphSynchronizer was called without any previously notified changes to the graph
• testKeepBranchFeaturesAfterSyncWithChanges in BranchGraphSynchronizerTest tests, if previously computed branch features still exist, after the sync() method of BranchGraphSynchronizer was called with previously notified changes to the graph. Since this is a current known issue, this test is ignored.

Resolves #279

[stefanhahmann]

@tinevez I am not sure, if this PR would contradict what you had in mind earlier, when you introduced the strategy to frequently update the branch graph to have it up-to-date in as many situations as possible?