Added support for adding and removing fields of filters onto statistics, currently only one available. Upon add then the filter type is stored onto the FilterModel for that statistic. Upon deletion it should be removed but is not currently. Also had some problems with genericity of MutableComponentView, so un-genericised it and made it into SelectableFilterview. That will become a head-ache surely, but it feels more simple to use the enums properly and will not need to cast as much.