Support tagging operator properties as 'advanced'

This will later be used to show advanced operator properties separate from
basic (as in non-advanced) ones in the UI.

Tagging a single operator property in C should be done via
`WM_operatortype_prop_tag()`. It does additional checks for type safety
that `RNA_def_property_tags()` doesn't do.

To avoid having to tag each advanced property individually, multiple
ones can be tagged by wrapping them into
`WM_operatortype_props_advanced_bein()` and
`WM_operatortype_props_advanced_end()` calls. It's also possible to only
call `_begin()`, all properties added after this will get tagged then.
In most cases this last approach should be sufficient.

Example of Python usage:
`my_float = bpy.props.FloatProperty(name="Some Float", tags={'ADVANCED'})`

source/blender/makesrna/intern/rna_access.c

@@ -727,6 +727,23 @@ bool RNA_struct_contains_property(PointerRNA *ptr, PropertyRNA *prop_test)
 	return found;
 }
 
+unsigned int RNA_struct_count_properties(StructRNA *srna)
+{
+	PointerRNA struct_ptr;
+	unsigned int counter = 0;
+
+	RNA_pointer_create(NULL, srna, NULL, &struct_ptr);
+
+	RNA_STRUCT_BEGIN (&struct_ptr, prop)
+	{
+		counter++;
+		UNUSED_VARS(prop);
+	}
+	RNA_STRUCT_END;
+
+	return counter;
+}
+
 /* low level direct access to type->properties, note this ignores parent classes so should be used with care */
 const struct ListBase *RNA_struct_type_properties(StructRNA *srna)
 {