[Bf-codereview] Implement "clear" method for most of collections (issue 5431052)

[sergey.vfx at gmail.com]

Reviewers: bf-codereview_blender.org,

Description:
This patch just adds .clear() method to most of RNA collections such as
modifiers list, vertex groups and so.

Method is NOT added to:
- Collections which doesn't have new/remove methods, also not added to
collections which have got new/remove commented in code.
- The whole animation-specific module. It's a bit complicated data
structure here, prefer to spend more time on this and make more tests.

Please review this at http://codereview.appspot.com/5431052/

Affected files:
   source/blender/editors/include/ED_mesh.h
   source/blender/editors/include/ED_object.h
   source/blender/editors/include/ED_view3d.h
   source/blender/editors/object/object_modifier.c
   source/blender/editors/object/object_vgroup.c
   source/blender/editors/space_view3d/view3d_edit.c
   source/blender/makesrna/intern/rna_curve.c
   source/blender/makesrna/intern/rna_meta.c
   source/blender/makesrna/intern/rna_nodetree.c
   source/blender/makesrna/intern/rna_object.c
   source/blender/makesrna/intern/rna_scene.c
   source/blender/makesrna/intern/rna_space.c