Issue #91: Cleaned up MCFileTreeRepository>>repositoryProperties and MCFileTreeRepository>>writeRepositoryProperties interaction.

  1. MCFileTreeRepository>>writeRepositoryProperties writes existing
     repositoryPropery dictionary and can be called at any time (like
     when a property is changed).
  2. MCFileTreeRepository>>repositoryProperties signals Error if the
     repository directory does not exist. If the .filetree file is not
     present, it is created with default repository properties.
  3. whenever MCFileTreeRepository>>directory: is called, the repository
     properties are reread from disk.
Merge branch 'issue_91' into gemstone2.4