Blender Git Loki

Fix T76734: Changing Envelope FModifier controlpoints missing update

Issue is that update functions defined in
`rna_def_fmodifier_envelope_ctrl` (namely `rna_FModifier_update`) are
actually never called.

This is because UI code for FCurve modifiers often does not use RNA
buttons but uses custom update functions (or non at all). For example,
rB9a88bd55903a did this for the generators, envelope control points did
not have this at all.

This is now changed to use RNA buttons for the envelope control points,
this could done for other non-RNA buttons as well to get rid of
'validate_fmodifier_cb()'.

Maniphest Tasks: T76734

Differential Revision: https://developer.blender.org/D7732