Datablock ID Properties

The absence of datablock properties "will certainly be resolved soon as the need for them is becoming obvious" said the [[http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.67/Python_Nodes|Python Nodes release notes]]. So this patch allows Python scripts to create ID Properties which reference datablocks.
This functionality is implemented for `PointerProperty` and now such properties can be created with Python.

In addition to the standard update callback, `PointerProperty` can have a `poll` callback (standard RNA) which is useful for search menus. For details see the test included in this patch.

Original author: @artfunkel

Alexander (Blend4Web Team)

Reviewers: brecht, artfunkel, mont29, campbellbarton

Reviewed By: mont29, campbellbarton

Subscribers: jta, sergey, campbellbarton, wisaac, poseidon4o, mont29, homyachetser, Evgeny_Rodygin, AlexKowel, yurikovelenov, fjuhec, sharlybg, cardboard, duarteframos, blueprintrandom, a.romanov, BYOB, disnel, aditiapratama, bliblubli, dfelinto, lukastoenne

Maniphest Tasks: T37754

Differential Revision: https://developer.blender.org/D113

source/blender/python/generic/idprop_py_api.c

@@ -43,6 +43,9 @@
 
 #include "python_utildefines.h"
 
+extern bool pyrna_id_FromPyObject(PyObject *obj, ID **id);
+extern PyObject *pyrna_id_CreatePyObject(ID *id);
+extern bool pyrna_id_CheckPyObject(PyObject *obj);
 
 /*********************** ID Property Main Wrapper Stuff ***************/
 
@@ -88,6 +91,11 @@ static PyObject *idprop_py_from_idp_group(ID *id, IDProperty *prop, IDProperty *
 	return (PyObject *)group;
 }
 
+static PyObject *idprop_py_from_idp_id(IDProperty *prop)
+{
+	return pyrna_id_CreatePyObject(prop->data.pointer);
+}
+
 static PyObject *idprop_py_from_idp_array(ID *id, IDProperty *prop)
 {
 	BPy_IDProperty *array = PyObject_New(BPy_IDProperty, &BPy_IDArray_Type);
@@ -148,6 +156,7 @@ PyObject *BPy_IDGroup_WrapData(ID *id, IDProperty *prop, IDProperty *parent)
 		case IDP_GROUP:    return idprop_py_from_idp_group(id, prop, parent);
 		case IDP_ARRAY:    return idprop_py_from_idp_array(id, prop);
 		case IDP_IDPARRAY: return idprop_py_from_idp_idparray(id, prop); /* this could be better a internal type */
+		case IDP_ID:       return idprop_py_from_idp_id(prop);
 		default: Py_RETURN_NONE;
 	}
 }
@@ -586,8 +595,15 @@ static IDProperty *idp_from_PyMapping(const char *name, PyObject *ob)
 	return prop;
 }
 
+static IDProperty *idp_from_DatablockPointer(const char *name, PyObject *ob, IDPropertyTemplate *val)
+{
+	pyrna_id_FromPyObject(ob, &val->id);
+	return IDP_New(IDP_ID, val, name);
+}
+
 static IDProperty *idp_from_PyObject(PyObject *name_obj, PyObject *ob)
 {
+	IDPropertyTemplate val = {0};
 	const char *name = idp_try_read_name(name_obj);
 	if (name == NULL) {
 		return NULL;
@@ -608,6 +624,9 @@ static IDProperty *idp_from_PyObject(PyObject *name_obj, PyObject *ob)
 	else if (PySequence_Check(ob)) {
 		return idp_from_PySequence(name, ob);
 	}
+	else if (ob == Py_None || pyrna_id_CheckPyObject(ob)) {
+		return idp_from_DatablockPointer(name, ob, &val);
+	}
 	else if (PyMapping_Check(ob)) {
 		return idp_from_PyMapping(name, ob);
 	}
@@ -732,6 +751,8 @@ static PyObject *BPy_IDGroup_MapDataToPy(IDProperty *prop)
 			return idprop_py_from_idp_float(prop);
 		case IDP_DOUBLE:
 			return idprop_py_from_idp_double(prop);
+		case IDP_ID:
+			return idprop_py_from_idp_id(prop);
 		case IDP_ARRAY:
 		{
 			PyObject *seq = PyList_New(prop->len);