If we parent with type PAR_ARMATURE (where vertexgroups are already
set up and named correctly according to the corresponding bones), we
still need an armature modifier. This just wasnt added.

In contrast to meshes [which add their armature modifier early in
ED_object_parent_set], grease pencil used to do this (adding the
armature modifier) in ED_gpencil_add_armature_weights.

Now split ED_gpencil_add_armature_weights in two:

• ED_gpencil_add_armature
• ED_gpencil_add_armature_weights (which calls ED_gpencil_add_armature)
• use ED_gpencil_add_armature for the PAR_ARMATURE case