Show the addon list in a ClassicAPI ScrollBox

The list built one row per addon in a ScrollFrame, hid the rows out of
view by hand and drove pfUI's own slider. A WowScrollBoxList keeps rows
only for the addons in view and recycles them while scrolling.

pfUIScrollBarTemplate (api/ui-widgets.xml) is a ScrollBarMixin bar with
the old slider's look: a thin track and a flat teal thumb that hides when
nothing scrolls. Its steppers exist for the mixin but stay hidden.

Rows read the addon's enabled state, so profiles enable and disable
addons directly instead of clicking every row's checkbox. A checkbox
now shows whether an addon is enabled; it showed whether it was loaded.

api/ui-widgets.xml

@@ -0,0 +1,31 @@
+<Ui xmlns="http://www.blizzard.com/wow/ui/">
+  <!--
+  pfUI's scroll bar for ClassicAPI's ScrollBox: a thin track with a flat
+  thumb. ScrollBarMixin needs Back and Forward steppers, so they exist but
+  stay hidden. Hook it up with ScrollUtil.InitScrollBoxListWithScrollBar.
+  -->
+  <EventFrame name="pfUIScrollBarTemplate" inherits="VerticalScrollBarTemplate" virtual="true">
+    <Size x="7"/>
+    <KeyValues>
+      <KeyValue key="hideIfUnscrollable" value="true" type="boolean"/>
+    </KeyValues>
+    <Frames>
+      <Frame parentKey="Track" setAllPoints="true" enableMouse="true">
+        <Frames>
+          <EventButton parentKey="Thumb">
+            <Size x="7"/>
+            <Layers>
+              <Layer level="ARTWORK">
+                <Texture setAllPoints="true">
+                  <Color r=".3" g="1" b=".8" a=".5"/>
+                </Texture>
+              </Layer>
+            </Layers>
+          </EventButton>
+        </Frames>
+      </Frame>
+      <EventButton parentKey="Back" hidden="true"/>
+      <EventButton parentKey="Forward" hidden="true"/>
+    </Frames>
+  </EventFrame>
+</Ui>