Scope tooltip cursor-follow to when shown; default it to smooth

The cursor-follow OnUpdate polled GetCursorPosition() 10x/second forever,
even with no tooltip visible. Rework it so the follower frame is created
once, hidden, and only shown while a tooltip is up -- an OnUpdate fires only
while its frame is shown, so the poll now runs solely during tooltip display.
Position the follower immediately on show to avoid a one-frame flash.

Also bump the tooltip_cursor throttle default from custom/10 FPS to the
fastest preset (50 FPS) so cursor tracking is smooth out of the box while
the Throttling tooltip knob stays available for low-end machines.

libs/libthrottle.lua

@@ -35,7 +35,7 @@ libthrottle.defaults = {
   nameplates_target    = "custom",
   nameplates_castbar   = "custom",
   nameplates_mass      = "custom",
-  tooltip_cursor       = "custom",
+  tooltip_cursor       = "fastest",
   chat_tab             = "custom",
   swingtimer           = "custom",
 }