Add Transition Type to IRequest

Based on #540 

**Includes the following changes**

- Add xml comments
- Rename NavigationType to TransitionType (matches Cef Api)
- Add [Falgs] attribute as TransitionType can have multiple values
- Import values from cef_transition_type_t (cef_types.h)
- Rename some of the values (slightly friendlier names)
- Separate file for enum (with license disclaimer)