event-hook: rewrite the hook priorities enum

Hooks need to have a priority relative to the event they are executed on,
so it does not make much sense to have all kinds of different priorities
based also on the event type and/or the module where they are defined.
Also, it wouldn't be acceptable to have such an enumeration on the public API.

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

@@ -191,11 +191,24 @@ 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,