Added a system for adding a "local" context in a UI layout.
This way you can define for example within a modifier panel
all operators to get the modifier in the context.
In the layout code:
uiLayoutSetContextPointer(layout, "modifier", &ptr)
layout.set_context_pointer("modifier", md)
In the operator:
ptr = CTX_data_pointer_get(C, "modifier")
md = context.modifier
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