* Remove entirely the hook priority numbers and use before/after dependencies
* Split the WpEvent code out of WpEventDispatcher
* Add methods on WpEvent to interface with it from the WpEventDispatcher.
As a bonus, we can now also implement tooling to inspect which hooks would
in theory run for an event and write tests around that
* Removed some internal debugging facilities and log calls, will redo it later.
* Using spa_list now for the list of hooks, to reduce the number of allocations
happening in the "hook collection" algorithm
* Switched some internal data to use g_new0 instead of g_slice_new0
* Added g_free to free WpEvent structures... surprisingly, we were leaking them
before
