Fix a slice error problem in parsing nestable async events
Merge pull request #637 from xunjieli/nestable_async_slice_error_fix

Fix slice error problem in parsing nestable async events
Move ObjectInstance and Snapshot registration to the base class
Group async slices using viewSubGroupTitle
Make AsyncSlices should use the Event type registry.
When a category is missing in Android parser, provide a default value instead of using undefined.
Add category support to event registrar
Add net/ and NetAsyncSlice subtype