Saved run serialization

[gered]

There seems to be some issue with serialization which causes saved runs created with Llanfair running on one JVM version where they won't load in the same version of Llanfair but running on a different JVM version. e.g. runs saved on Java 8 won't load in Llanfair running on Java 7. I'm assuming the issue is caused by something with the ImageIcon serialization specifically, but I guess it could be any Java runtime / Swing class which we just rely on XStream's automatic serialization behaviour.

[gered]

Can fix this at the same time as #11.