Add default preference for max tabs in staff client.

Add default preference for max tabs in staff client.

By default, this is set to 0 for unlimited tabs. Including this setting
will allow staff client packagers to more easily assign a limit if desired.

Some reasons for adding a tab limit include:

  1) avoiding memory exhaustion - opening too many tabs can be bad
  2) preserving staff sanity - do not have too many tabs open at once

Signed-off-by: Ben Shum <bshum@biblio.org>