Support global macros via the importvariables widget

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

core/ui/PageTemplate.tid

@@ -4,6 +4,8 @@ title: $:/core/ui/PageTemplate
 tw-page-container tw-page-view-$(themeTitle)$ tw-language-$(languageTitle)$
 \end
 
+<$importvariables filter="[[$:/core/ui/PageMacros]] [all[shadows+tiddlers]tag[$:/tags/Macro]!has[draft.of]]">
+
 <$set name="themeTitle" value={{$:/view}}>
 
 <$set name="currentTiddler" value={{$:/language}}>
@@ -37,3 +39,5 @@ tw-page-container tw-page-view-$(themeTitle)$ tw-language-$(languageTitle)$
 </$set>
 
 </$set>
+
+</$importvariables>