option to enable/disable 'experimental' options in Jalview's UI

When new features are first developed, they may not always be fully tested by the time a release happens, or there may be known issues which make the feature unsafe for general use.

Rather than hold back these features from released code, part-finished features could be made available via an 'experimental' mode - a user preference that when set, causes the UI to include additional widgets that provide access to these unstable functions.