upgrade varna to version that supports version independent state storage/retrieval

The current version of VARNA (3.8b) introduced into Jalview 2.7.x, and all versions prior, use a java object serialization to store a snapshot of the current datamodel and visual display settings. These serializations are not backwardly compatible, so cannot be used to save the state of the VARNA display within a jalview project archive.

Once a version - independent mechanism is available, the version of VARNA used by jalview should be upgraded and Jalview2XML extended to support the use of this mechanism to save the layout and visual attributes of a VARNA display embedded within Jalview.