use more api functions more (some vertex group editing functions were copied about), also make some functions int oapi calls.
- remove defgroup_find_index(), use BLI_findlink instead since they both work the same way.
- move static function getNearestPointOnPlane() to BLI_math api function closest_to_plane_v3()
- ED_vgroup_give_parray() added option to return an array where unselected verts are NULL (simplifies code & works for lattice when it didn't before).
- more consistant error checking of ob->actdef.
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