Replace HookAddonOrVariable with an EventUtil-based HookAddon

The polling lurker frame is gone. HookAddon gates on
C_AddOns.DoesAddOnExist, waits for PLAYER_LOGIN with
EventUtil.ContinueOnPlayerLogin, then runs on
EventUtil.ContinueOnAddOnLoaded. Addons loaded at startup still hook
only after every pfUI module and skin exists and after the addon's
own VARIABLES_LOADED setup. Load-on-demand addons still hook on their
ADDON_LOADED.

The "or variable" form is gone. Callers that passed a global now pass
the addon that defines it: TheoryCraft, SuperMacro, DPSMate and
Turtle_GroupUI. DPSMate names its windows from its settings, so its
hook now checks for DPSMate_DPSMate itself, as the variable lookup
used to. eqcompare's Guda check uses HookAddon too.

skins/blizzard/mail.lua

@@ -28,7 +28,7 @@ pfUI:RegisterSkin("Mailbox", function ()
       end)
       skin:RegisterEvent("MAIL_SHOW")
 
-      HookAddonOrVariable("Mail", function()
+      HookAddon("Mail", function()
         skin._Mail = true
         for i = 1, 21 do
           local button = _G["MailAttachment"..i]