Standardize the base for scene methods/events

[virn]

Currently, for scene-load event scene is returned as 0-based, while scene-add, scene-delete events and setActiveScene method are at 1-base.

Let's review why we switched to 1-base on the later events and methods @SML-MeSo @Matrim-Cauthon and discuss what base we should use and update to.

[mikeybanez]

My two cents here:

I'm in favor for 0-based. I don't remember if there was an original reason for a 1-based system for setActiveScene, but since Scene objects are returned through an array, it makes sense to just use 0-based indexes for everything. I think it's a fair assumption to say that many who use xjs already know a bit of programming and should already be used to point to "Scene 1" at index 0. This helps avoid a lot of + 1s in plugin code.

I'll also move this to a 3.0.0 milestone because of its breaking nature, even if there is no schedule for a version 3 yet.

[SML-MeSo]

add to this item changes for addToScene methods of the classes within the system directory