Add "hasGeneTrack" field to session files, use that to determine whether gene track should be loaded

IGV-1370
Show a cancellable indeterminate progress dialog when loading alignments

GH issue #28
Set hasGeneTrack to be true by default
Merge pull request #52 from broadinstitute/CancelLoaders

Show a cancellable indeterminate progress dialog when loading alignments
Merge pull request #51 from broadinstitute/GeneTrackRemoval

Add "hasGeneTrack" attribute to session files