Add ClipboardUpdateEvent to Event
Add UnknownEvent to Event

An unknown event from the SDL2 library. Rather than throwing an error,
we'll return an UnknownEvent.

This prevents program crashes due to linking against newer versions
of SDL2 that may have introduced new event types, or due to missing
bindings.
Update cabal file to version 1.1.2
Add source-repository stanzas to the cabal file