Access to Groups and Sources via their names

[tischi]

@tpietzsch @tibuch It would be really helpful to be able to write code like this:

bdv.getViewerPanel().getState().getVisibilityAndGrouping().setGroupActive( (String) nameOfGroup, boolean );
bdv.getViewerPanel().getState().getVisibilityAndGrouping().setSourceActive( (String) nameOfSource, boolean );

Without this one has to constantly keep track of how the respective names map to the respective indices.