event-dispatcher: refactor to use before/after dependencies on hooks

* 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

modules/module-lua-scripting/api/api.lua

@@ -191,24 +191,11 @@ local Feature = {
   },
 }
 
-local HookPriority = {
-  LOWEST = -500,
-  ULTRA_LOW = -300,
-  VERY_LOW = -200,
-  LOW = -100,
-  NORMAL = 0,
-  HIGH = 100,
-  VERY_HIGH = 200,
-  ULTRA_HIGH = 300,
-  HIGHEST = 500,
-}
-
 SANDBOX_EXPORT = {
   Debug = Debug,
   Id = Id,
   Features = Features,
   Feature = Feature,
-  HookPriority = HookPriority,
   GLib = GLib,
   I18n = I18n,
   Log = WpLog,