Error and event type enumerations

The enumerations in types.h define error code constants as well as media source and tracksession event types.

To make your Qt app code more readable, you can use the error code constants when inspecting function return values. The MediaSourceEventType and TrackSessionEventType enumerations define constants that are encoded in some of the signals emitted by QPlayer objects. Qt slots connected to these signals can then check for specific event type constants, which further improves code readability.