Anim: RNA, mark armature.collections as editable

By setting the RNA property flag `PROP_EDITABLE` on `armature.collections`,
the UIList understands that it's editable and will show "Double click to
rename" in the tooltip.

Note that this does not make the array itself editable; assignment like
`collections[0] = collections[1]` will still be refused.

source/blender/makesrna/intern/rna_armature.cc

@@ -1980,6 +1980,7 @@ static void rna_def_armature(BlenderRNA *brna)
                                     nullptr, /* TODO */
                                     nullptr);
   RNA_def_property_ui_text(prop, "Bone Collections", "");
+  RNA_def_property_flag(prop, PROP_EDITABLE);
   RNA_def_property_override_funcs(
       prop, nullptr, nullptr, "rna_Armature_collections_override_apply");
   RNA_def_property_override_flag(