Remove alpha and beta categories. Mark nodes in development by frames.

[Durman]

Addressed problem description

It's implementation of this proposal #4685 Now nodes in development can be marked by frames. To do this alpha or beta icon should be added to a node class.

class Node:
    sv_icon = 'SV_ALPHA`  # or 'SV_BETA`

I moved some (seems to me as redundant) nodes from removed categories to old nodes. Some nodes marked as Alpha/Beta nodes. I does not have strong option about whether I've made changes correct so if someone has any suggestions they are welcome.

Preflight checklist

• [x] Code changes complete.
• [x] Code documentation complete.
• [x] Documentation for users complete (or not required, if user never sees these changes).
• [x] Ready for merge.

[portnov]

Maybe instead of changing icon it's better to have special class property for this? like

class MyNode:
    sv_alpha = True

[Durman]

Using icons has extra advantages. It's clear from menu that a node is in Alpha/Beta stage.