Created by: portnov
In Blender 2.80, the T panel does not support tabs; so there are no automatically registered panels for node categories under T panel.
This adds the following options to preferences:
- Display node buttons:
- Under T panel (with popup menu for categories selection)
 - Under N panel (categories are selected with tabs)
 - Do not show (default)
 
 - Display icons only: if set, then for nodes that have icons, only icons will be shown, to save screen space. Checked by default.
 - Number of columns to show in "icons only" mode. 4 by default.
 
With new icons from #2536:
Preflight checklist
Put an x letter in each brackets when you're done this item:
- 
Code changes complete.  - 
Code documentation complete.  - 
Documentation for users complete (or not required, if user never sees these changes).  - 
Manual testing done.  - 
Unit-tests implemented.  - 
Ready for merge.  


