New export button appears as a page control toolbar button, a tiddler
toolbar button, and a button in the advanced search filter tab.
Initially supports exporting as static HTML, CSV, JSON or `.tid` file.
Still to do:
* Made the exporter descriptions translatable
* Hide the export button by default
* User docs
* Cleaning up the existing templates (eg,
`$:/core/templates/alltiddlers.template.html` should work by
transcluding `$:/core/templates/exporters/Static`)
* Docs for the new macros `exportButton`, `csvtiddlers` and
`jsontiddlers`
Issues:
* OS X Numbers refuses to open CSV files that have been generated in
Chrome, because it thinks they’ve been downloaded from the Internet.
Firefox works OK
* The export button won’t work within the tiddler info panel, or from
the *more* popup (this is because we don’t support nested popups)
