Move the CSS macros into global macros, and allow the colour macro to
fallback to the vanilla palette if the required colour isn’t found in
the current palette.
The new importvariables widget imports macro/variable definitions from
the specified tiddlers and makes them available to its children.
Allows us to split PageMacros up into separate tiddlers.
We still support loading macros from $:/core/ui/PageMacros to help
people upgrading.
Fixes#644 and #559