Refactor: Simplify ID Property freeing

This also makes `IDP_CopyProperty` the "opposite"
of `IDP_FreeProperty`, which is what I'd expect.

Two refactoring steps:
* rename IDP_FreeProperty to IDP_FreePropertyContent
* new IDP_FreeProperty function that actually frees the property

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4872

source/blender/python/generic/idprop_py_api.c

@@ -601,7 +601,6 @@ static IDProperty *idp_from_PyMapping(const char *name, PyObject *ob)
     pval = PySequence_GetItem(vals, i);
     if (BPy_IDProperty_Map_ValidateAndCreate(key, prop, pval) == false) {
       IDP_FreeProperty(prop);
-      MEM_freeN(prop);
       Py_XDECREF(keys);
       Py_XDECREF(vals);
       Py_XDECREF(key);
@@ -690,7 +689,7 @@ bool BPy_IDProperty_Map_ValidateAndCreate(PyObject *name_obj, IDProperty *group,
       prop->prev = prop_exist->prev;
       prop->next = prop_exist->next;
 
-      IDP_FreeProperty(prop_exist);
+      IDP_FreePropertyContent(prop_exist);
       *prop_exist = *prop;
       MEM_freeN(prop);
     }