PyAPI: extend save/load handlers, optionally take a filepath argument

Add a filepath argument to load/save pre/post.
Also add save_post_failed and load_post_failed handlers so it's always
possible to for the pre handlers to run a matching post action.

This makes it possible to know the filepath of the blend file mean
loaded/saved as well as supporting running an action when load/save
operations fail.

When loading and saving the startup-file, the path argument is set to
an empty string.

Details:

New RNA types were added to support storing primitive values in
PointerRNA. Primitive{String/Int/Float/Boolean}RNA. These will likely
only be used in some limited cases, in the case of BKE_callback_exec it
allows strings to be included as part of the PointerRNA **pointers
argument.

Ref !104769.

source/blender/python/intern/bpy_rna.c

@@ -7407,6 +7407,27 @@ PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr)
   return (PyObject *)pyrna;
 }
 
+PyObject *pyrna_struct_CreatePyObject_with_primitive_support(PointerRNA *ptr)
+{
+  if (ptr->type == &RNA_PrimitiveString) {
+    const PrimitiveStringRNA *data = ptr->data;
+    return PyC_UnicodeFromBytes(data->value);
+  }
+  if (ptr->type == &RNA_PrimitiveInt) {
+    const PrimitiveIntRNA *data = ptr->data;
+    return PyLong_FromLong(data->value);
+  }
+  if (ptr->type == &RNA_PrimitiveFloat) {
+    const PrimitiveFloatRNA *data = ptr->data;
+    return PyFloat_FromDouble(data->value);
+  }
+  if (ptr->type == &RNA_PrimitiveBoolean) {
+    const PrimitiveBooleanRNA *data = ptr->data;
+    return PyBool_FromLong(data->value);
+  }
+  return pyrna_struct_CreatePyObject(ptr);
+}
+
 PyObject *pyrna_prop_CreatePyObject(PointerRNA *ptr, PropertyRNA *prop)
 {
   BPy_PropertyRNA *pyrna;