* Added an optional array of MDeformModifier pointers, to avoid another search based on defgrp_idx.
* Split out "add/remove verts from vgroup" functions, preparing their move to deform.c (if their current form is validated!).
*Added Smooth/Sharp/Root/etc. mappings to WeightVGEdit modifier, in addition to custom curve one.
*Added Smooth/Sharp/Root/etc. mappings to WeightVGProximity modifier, without the custom curve one!
*Factorized the common mapping code into MOD_weightvg_util.