Skin the Challenges frame's Challenges tab

The tab is the Trials tab again in every detail that matters: the same
three children under the same suffixes, the same store divider as its
list container's only region, the same silver-button filter, and its
scroll arrows named off the scroll frame rather than off the bar. So it
takes the same four calls, sharing the divider constant rather than
copying it.

Everything exists before the tab has ever been opened, so it goes
through SkinContents with the rest and needs no hook of its own.

Modules/ChallengesFrame.lua

@@ -129,8 +129,8 @@ local function UpdatePanelTop(frame)
 end
 
 -- Per-tab contents. Each of the six tabs owns its own copy of these widgets
--- rather than sharing one set, so they're skinned per tab; Trials and Rewards
+-- rather than sharing one set, so they're skinned per tab; Trials, Rewards and
--- are done and the rest follow the same shape.
+-- Challenges are done and the rest follow the same shape.
 --
 -- Nothing about one tab's naming carries to the next: the Trials search box is
 -- "...TrialsTabSearch" and its filter a "...FilterDropDown", where the Rewards
@@ -311,6 +311,31 @@ local function SkinStoreTab()
 	end
 end
 
+local CHALLENGES_TAB = FRAME_NAME.."ChallengesTab"
+local CHALLENGES_LIST = CHALLENGES_TAB.."Challenges"
+local CHALLENGES_SCROLL = CHALLENGES_LIST.."ScrollFrame"
+
+-- The one tab whose naming does carry over from another: the same three
+-- children as Trials, under the same suffixes, with the same store divider as
+-- its list container's only region, the same nine "UI-Silver-Button" slices and
+-- caret on its filter, and its arrows named off the scroll frame rather than
+-- off the bar (probed). So it takes the Trials treatment call for call -- see
+-- SkinTrialsTab above for why each of the four is the call it is.
+--
+-- The divider constant is shared rather than copied: it is the same 2px rule out
+-- of the same file, not a second one that happens to match today.
+local function SkinChallengesTab()
+	Skin:SearchBox(_G[CHALLENGES_TAB.."Search"])
+	Skin:StripArtByFile(_G[CHALLENGES_LIST], LIST_DIVIDER_ART)
+	Skin:Dropdown(_G[CHALLENGES_TAB.."FilterDropDown"])
+	Skin:ScrollBar(
+		_G[CHALLENGES_SCROLL.."ScrollBar"],
+		_G[CHALLENGES_SCROLL.."ScrollBarThumb"],
+		_G[CHALLENGES_SCROLL.."ScrollUpButton"],
+		_G[CHALLENGES_SCROLL.."ScrollDownButton"]
+	)
+end
+
 -- The close button is anchored inside the frame's own top-right corner, which
 -- stopped being the corner the player sees once the panel grew up over the title
 -- band -- it ends up floating a title's height below the top edge. Re-anchored
@@ -342,6 +367,7 @@ local function SkinContents()
 	AnchorTabRow()
 	SkinTrialsTab()
 	SkinStoreTab()
+	SkinChallengesTab()
 end
 
 local function SkinFrame(frame)