[Xerte] Xerte3: on Mac - try now

[Julian Tenney]

In the SVN now are the Xerte3 file and the Xerte3.app folder.

The code should test for sandbox type and show an error on MacOS if it
isn't 'localTrusted'. Let me know if that gets shown?

All the menus should work now - you should be able to create a new
project and open it.

Import media should work too.

Compile stylesheets probably doesn't yet. First things first: we need to
get your preview working.

We can try older versions of Zinc as a Mac user reported some issue with
sandboxType in the forums on the new builds...