Fixes and improvements to JointCollisionEvents behavior

[NGenesis]

Updated JointCollisionEvents behavior:

• Corrected issue with scene object relying on global variable during behavior initialization.
• Updated behavior to check for and reuse existing skeleton object in scene during behavior initialization.
• Updated jointcollision event to provide an array of joints that were included in the intersection union.
• Added jointcollisionenter (not colliding -> colliding) and jointcollisionleave (colliding -> not colliding) events which are fired only once when a change in collision state has occurred.
• Corrected issue with collision events not being dispatched with target and currentTarget properties.
• Corrected issue with collision events being fired on invisible objects.

[evhan55]

Oh wow, this all sounds amazing :satisfied:. I just submitted a tiny fix to the documentation but I will rescind that PR and maybe help flesh out the docs whenever this merges! I think jointcollisionenter and jointcollisionleave are great additions, I had something like that initially but took them out because I wasn't treating it very cleanly.

[brianpeiris]

Thanks @NGenesis! I'll take a closer look at this later today. @evhan55 You're welcome to help review the changes if you have time.

[evhan55]

Documentation seems great now!

[brianpeiris]

And thanks for the documentation as well! I know jsdoc can be tricky to work with.