Scene handler

[Durman]

Addressed problem description

Closes #4283. It should update nodes reading data from Blender upon changes in the scene.
I feel this make work with Sverchok more comfortable in case when workflow include reading data from Blender.
Update live operator can be removed.

Preflight checklist

• Code changes complete.
• Code documentation complete.
• Documentation for users complete.
• Manual testing done.
• Ready for merge.

[Durman]

I removed AnimatableNode class. Now it's possible make a node to be animatble in this way.

class Node:
    is_animation_dependent = True

Also, instead of draw_buttons, sv_draw_buttons should be used now. It will let to draw extra buttons in the node UI.

Also it's possible to add is_scene_dependent = True property. In this case the node will be update on user changes in the scene.

[rendetto]

I just did a quick test on one of my heaviest scenes and it works perfect.
Great solution that makes a ton of difference for my workflow.

Thank you @Durman!

[Durman]

@rendetto I'm glad that it's useful =)