Update system requirements

Created by: Durman

I'm going to create list of desirable features which new update system should include. What can be included in this list is not obvious and can be discussed.

Performance issue.:
- Don't recalculate what already was calculated:
  - Nodes should keep results of their calculations.
  - All changes (in parameters, inputs, data from Blender scene ...) should be tracked.
  - Manually prohibition of recalculation (freezing mode)
- Don't calculate if result is need for nobody:
  - Create output nodes which cause work of other nodes (viewers).
  - Mode for output nodes switch on/off of showing of calculation results
User should understand what's going on:
- Use annotations for showing error and other messages.
- Color system for showing error nodes and different modes or whatever.
- Showing performance information.
Performance of workflow:
- supporting of node subtrees
- pass data without changes throu node (bypass mode. It is not possible for all nodes)