Fixed a side-effect of 03519c14b0 from
@matabele whereby the current tiddler is overwritten by the
`<$tiddler>` widget, which means that it is not accessible when
rendering the caption of the tab.
This causes a problem with eg the “Add Plugins” modal where it breaks
the count on the tabs.
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
