Refactoring of highlevel workspace code

Move all workspace related code out of Pipeline() and into the
frontend App() object.

Some changes in transition here include:

  o Workspaces() object methods for looking up and deleting workspaces
    now take an element name instead of an element.

  o Share code for partial App() initialization between the
    `workspace close` and `workspace list` commands

  o No longer require that an element exist in the project
    in order to close a workspace

This fixes issue #249