Group frame skins under their own settings tab

The CoA options were a flat row of tabs, one per feature, which doesn't
survive the number of frame skins now landing. Skins get their own tab,
first in the row, laid out as a vertical tree so each skinned frame is one
entry rather than one more horizontal tab. Extra Action Button and Instance
Swap move under it.

Each skin also gets a master enable toggle. Skinning is one-way -- the
native art is stripped and replaced in place -- so turning one off can only
take effect on the next load, hence the reload prompt and the gate in
CoA:Initialize rather than an attempt to undo the skin live. Existing users
see no change: both default to on.

LayerPicker's UIDropDownMenu_AddButton wrapper is hooked at file scope,
outside that gate, so it checks the toggle itself. That one does respect
the switch immediately.

Modules/LayerPicker.lua

@@ -23,10 +23,15 @@ function CoA:UpdateInstanceButtonFont()
 	UpdateFont()
 end
 
+-- Hooked at file scope, so unlike the rest of the skin it stays live even when
+-- the skin is off -- check the toggle here instead. Reset Position only makes
+-- sense while the frame is where the server put it; once we've handed it to a
+-- mover the entry does nothing useful.
 do
 	local orig_AddButton = UIDropDownMenu_AddButton
 	UIDropDownMenu_AddButton = function(info, level)
-		if info and info.text == "Reset Position" and UIDROPDOWNMENU_INIT_MENU == LayerPickerFrameDropDown then
+		if CoA.db and CoA.db.profile.skins.instanceSwap
+		and info and info.text == "Reset Position" and UIDROPDOWNMENU_INIT_MENU == LayerPickerFrameDropDown then
 			return
 		end