Add back custom properties to nodes, and properly expose them in the UI
as such (new 'Custom Properties' panel for nodes in the the Node
Editor).
This shows like any other custom property panel in the UI, in the Node editor:
<img width="424" alt="image.png" src="attachments/adb50815-0db0-447f-ac3b-7860d9a3e3fc">
Although not initially intended for this, nodes 'mixed' IDProperties
were quite widely used by py/extensions developers to store per-node
data.
During the initial IDProperties split between User-defined (aka custom
data) and system-defined (runtime RNA properties), this was not known,
so decision was made to only keep system properties for Nodes. The fact
that these properties were not exposed at all in UI also played a role
in that decision. See also discussions in #141042.
This commit instead splits system properties out into their own new
storage, and keep existing ones for user-defined/custom properties
(as done for other IDProperty owners, like IDs, bones, etc.).
Pull Request: https://projects.blender.org/blender/blender/pulls/145424
329112c4d3