Refactor random functions into own MinSG extension

[eikel]

Initially created by @eikel as #673:

For the AGVS, I implemented some random generator functions (e.g. generateViewSpaceDirectionSample). Maybe it is a good idea to put them into their own MinSG extension to be usable by other extensions, too. I think about simple helper functions – something like Vec3 generateRandomDirection() – that can be embedded into more complex algorithms. Maybe they can even be moved to Geometry. Two applications that come into my mind:

1. Generate a path with random waypoints (random position and random viewing direction).
2. Generate random sample positions on a sphere (for SVS).