Add a demo control panel page for the Starlight theme

If you open the control panel and then use the tools sidebar to switch
themes to and from Starlight, you'll see this tab dynamically appear
and disappear

themes/tiddlywiki/starlight/themetweaks.tid

@@ -0,0 +1,5 @@
+title: $:/themes/tiddlywiki/starlight/themetweaks
+tags: $:/tags/ControlPanel
+caption: Star Tweaks
+
+Demo of a control panel tab dynamically loaded with a theme.