Small API enhancements

[Daniels118]

Dear Emmanuel,
this is my wishlist for Christmas:

a.1) Make Object3DBranchFactory a singleton, so that all components share the same instance and can benefit of the added factories. An alternative to the singleton could be to add a method like "getObject3DFactory" to the HomeController3D.
a.2) Add a method to Object3DBranchFactory to allow the addition of new Object3D factories. It *could* work like ExtensibleObject3DFactory that you can find in my plugins (Wirings, 3D dimesnion lines).
Ticket #1142

b) in HomeComponent3D make the bottomHalfSphere of the background always visible and add to it the ColoringAttributes with the proper capabilities to change the color. Having a way to change it through the public API would be nice, but not required.
Ticket #1143

c) maybe you could integrate the pan of the 3D view functionality? [:p]

I know you have some concerns about copying 3rd party code due to licensing. If this is the case, I would be available to yield the copyright of the required code.

[Puybaret]

Thanks for the suggestions. Nice things to think about, probably next year.