Here we are overly testing the same functionality which does
not require source specific support, e.g. lets not test resetting
of workspaces for every kind of repo, since we already test opening
and closing of workspaces for every repo kind, this is redundant
and slowing down tests.

Guard against regressions of issue #249

This was never particularly useful, there is no circumstances
under which a workspace needs to be deleted, and a cache key
invalidated, in the course of a session.

A workspace is deleted only atomically as a part of `bst workspace close`,
which does not even load a pipeline anymore, so the pipeline state need not
be adjusted in this case.

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

Created partial_init() function to do a partial initialization
which does not require loading the Pipeline, which is more expensive.

This is useful for some bst commands which dont operate on a pipeline.

Since the CLI and the App object was split up, we should name
the file after the object which it implements.

Seems that using `~` conflicts with some markdown when trying to
paste build logs in, e.g. gitlab issues.