disable GUI when saving state to prevent user's actions from corrupting window state

When Jalview is performing some lengthy data storage task like saving the current GUI state, it's possible for the user to interact with the windows during the storage procedure, and potentially corrupt the stored information.

We can do one of two things here - one is to create a transparent pane over the top level window (ie the desktop) to prevent any GUI events being handled during the save, and the other is to somehow make the datamodel read only during the save. The latter is preferable, but probably technically difficult.