Follow target-of-target frames with UNIT_TARGET

A frame whose unit is another unit's target (targettarget,
targettargettarget, pettarget, focustarget, partyNtarget) has no token
that unit events name, so it polled: bars every 0.2s, a full aura and
portrait rescan every second, and a GUID compare to notice swaps.

Such a frame now registers its unit events by the unit's GUID, which
every unit event also fires with, and UNIT_TARGET by the GUID of every
link of its chain. A link retargeting re-points the frame at once and
queues one full refresh; the ticks are gone. The 0.2s visibility timer
stays for them, since nothing reports a unit going out of sight.

Nameplates register UNIT_TARGET on their own token, so a mob turning to
someone else recolours its combat state immediately instead of after
the colour cache and data tick run out.

Drop pfScanActive and the namebuf delay: both guarded against the old
target-swapping range scan, which no longer exists. focus.lua's
instantRefresh was never read.

modules/targettarget.lua

@@ -2,7 +2,7 @@ pfUI:RegisterModule("targettarget", function ()
   -- do not go further on disabled UFs
   if C.unitframes.disable == "1" then return end
 
-  pfUI.uf.targettarget = pfUI.uf:CreateUnitFrame("TargetTarget", nil, C.unitframes.ttarget, .2)
+  pfUI.uf.targettarget = pfUI.uf:CreateUnitFrame("TargetTarget", nil, C.unitframes.ttarget)
   pfUI.uf.targettarget:UpdateFrameSize()
   pfUI.uf.targettarget:SetPoint("BOTTOM", UIParent , "BOTTOM", 0, 125)
   UpdateMovable(pfUI.uf.targettarget)