mgContainer id could contribute to dataPath

[mryellow]

Currently child items support dot-notation in their id field.

{"type": "mgContainer", "items": [
	{"type": "mgText", "id": "obj.field1", "title": "Field 1"}
]}

It may be desirable to use wrapper mgContainer as context for $dataPath.

{"type": "mgContainer", "id": "obj", "items": [
	{"type": "mgText", "id": "field1", "title": "Field 1"}
]}