In 13fac109 the node panels got support for individual option button
callbacks, but these were not included in the node editor side bar.
Only the older top-level buttons are drawn there.
The panel structure is currently not accessible in python since it is
part of the `NodeDeclaration` system. To draw node input sockets and
buttons in the correct panel order as they appear on the node, a new
template function `uiTemplateNodeInputs` has been added. This iterates
over declared panels and their contents in the appropriate order and
draws the buttons before sockets in the same panel.
Pull Request: https://projects.blender.org/blender/blender/pulls/116936
53e34dba80