Introduce a PageMacros tiddler containing a tabs macro

This is a hacky way of having macro definitions that behave as if they
are global to the wiki.

The new tabs macro itself is cool, though: neatly encapsulates most of
the gubbins around a tab control, and the rendering is quite easy to
style.

core/wiki/sideBarTabSet.tid

@@ -1,3 +1,3 @@
-title: $:/state/sideBarTabSet-{$:/core/ui/SideBar|$:/core/ui/SideBar||}
+title: $:/state/sideBarTabSet-{$:/core/ui/SideBar|$:/core/ui/SideBar||}{$:/core/ui/PageTemplate|$:/core/ui/PageTemplate||}
 
 openTab