CanvasGroup alpha also affects child GameObjects alpha, whereas Image alpha only affects the Image itself

The FactExplorer Window shows other facts that reference the selected facts and also shows facts referenced by the selected fact.

Additional changes:
- reverted SortingOrder of MathMenue Canvas back to 0 (was 10)

WorldFactInteraction:
Hovering over a worldspace fact in mathmode now shows the fact display. This display can be dragged into scrolls like facts from the fact display.

Additional changes:
- WorldFactInteraction added to Mathmode canvas, as it should only be active if the canvas is active
- Sorting order of Math canvas increased to 10 (from 0), so that dragged facts will always be displayed on top
- DisplayFacts Dictionary is now static and thus available from other classes that need the fact display (this is not ideal and should maybe be extracted)
- DragHandling: dragged variable now public, as this is of interest to other classes