Skin the CoA talent frame's Character Advancement tab

Flattens the custom talent window into the ElvUI look: frame chrome, close
buttons, tabs, bottom bar, dropdowns and their popups, the list scrollbars,
and the spec choice cards' action buttons. The talent tree itself is left
alone -- its node borders encode rank and availability.

Most of the work is in keeping the skin applied. The frames restore their
native art on refresh in three different ways, and none of them fire an
event to hook:

- The scroll arrows re-show a region StripTextures had hidden, so the arrow
  buttons re-hide it from their own update, matching by file so ElvUI's
  panel pieces (which are regions of the same button on this client) survive.
- The pooled menu rows put their hover and selection art back when a scroll
  refills them, without firing OnShow, so the strip runs from the row update
  behind a cheap texture compare.
- The talent frame's close button keeps its absolute frame level while
  TreeView rides along when the frame is raised, so the level bump re-applies
  on every show.

Row fills and the selection edge are textures on the row rather than an ElvUI
backdrop: the backdrop is a child frame and covered the row's icon and text at
every frame level available to it.

ElvUI_CoA.toc

@@ -14,3 +14,4 @@ Modules\ExtraActionBar.lua
 Modules\LayerPicker.lua
 Modules\DispelHighlight.lua
 Modules\ClassResources.lua
+Modules\TalentFrame.lua