Fix memory leakage

[abooij]

I have planned for a while to fix a memory leakage in the current implementation. Specifically, the callback mechanism (from the C wayland lib into haskell code) works by storing a struct of callback functions on the C side, but this struct is currently not getting destroyed when the object itself gets destroyed.

This can be fixed by using the void* user_data storage (make sure not to expose this functionality to the haskell API).