should provide API to access state and geometry data saved by saveState/saveGeometry

Currently we provide saveState and saveGeometry to save the state data of the widgets, and also provide restoreState to decode the data from the file. However it would be convenient if we can provide API to access the data and decode it into a readable format, so that it's easy to implement something like the property editor in designer.
But till now the data format is undocumented and there is no API to read/decode the data, which makes it difficult.