Fix problem with CSS macros

In practice the macros are always invoked in inline mode, meaning that
the triple braces weren’t getting parsed correctly. Now we switch to
inline code
Add group headings to default search results
Update savers to specify variables, including filename

Now the `tm-download-file` and `tm-save-file` messages use the hashmap
to specify variables to be applied when rendering the tiddler.

We also add a convention that the variable “filename” is used to
specify a filename for the download.
Updated export button

Made the border a bit thicker
Fix search results to be in block mode
First pass at export button

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)