_workspaces.py: Add WorkspaceProject and WorkspaceProjectCache objects

WorkspaceProject contains all the information needed to refer back to a
project from its workspace (currently this is the project path and the
element used to create this workspace)

This is stored within a WorkspaceProjectCache object, which keeps
WorkspaceProjects around so they don't need to be loaded from disk
repeatedly.

Workspaces has been extended to contain the WorkspaceProjectCache, and
will use it when opening and closing workspaces.

Context has been extended to contain a WorkspaceProjectCache, as there
are times when we want to use it before a Workspaces can be initialised
(looking up a WorkspaceProject to find the directory that the project is
in)

This is a part of #222