Added a SoundManager Class and a Volume Slider to the options menu that can be used to change the volume of all sounds.

The toggle button will be used to filter for favorited facts

Facts can be marked as favorite on middle mouse button click.
Favorised facts are indicaded by a yellow star in the top right corner.
Other fact displays will be updated accordingly and newly spawned fact displays will show the favorite star correctly.

Additional changes:
- Refactored Prefabs/UI/Facts project structure

Implemented UILine

Parent facts are connected to the main fact by a direct dashed line whereas child facts are connected by a jagged continuous line.

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