simple admin UI

A start, and useful, but not enough to call #479 complete.

Loads and displays the filesystem tree, but does not yet
handle running queries, although the UI is stubbed out.

If there is any configuration problem, it should go straight
to the configuration edit UI and not allow any bad connection
information to be saved.

Connection testing is pretty simple here, and only checks that
an `ls` call returns something.
re-work config UI with more helpful interaction

No more editable table, and no more entering the connection
URI directly. Now the URI is broken out into fields for
editing, with real-time feedback for errors that can be
statically detected. When Save is clicked, a test is
performed, and the action does not continue if any error
occurs.

The SlamData path for a mounting is never shown until there
is more than one mounting. That goes for the overview table,
and also for the edit dialog.

The SlamData port is moved to the bottom of the dialog.

Most of the commonly-used text-editing keystrokes for
Mac OS X are now bound, including cut/copy/paste.

All of the config errors that are easy to provoke are now
detected and a nicer alert message is shown.
Merge pull request #495 from mossprescott/ready/479

Admin UI, simplified and more error proof