Reapplied 2 changes because lost in last change:
- Implemented changing of editor data as stack is sliced.
- A bit of tidying up.
PlotImageEditor now implements IEditorExtension, and plottingSystem is
replaced with getPlottingSystem() thus it is more reusable.
Merge branch 'master' of git@github.com:DawnScience/dawn-ui.git