Integrating BuildWrangler.
Activated build file library menu and distribution file library menu.
Renamed BuildFileWrangler and DistributionFileWrangler.

The old names were BuildWrangler and DistributionWrangler.
Added FileWrangler. Rewired wranglers.

Wrangler class hierarchy is now this:

    Wrangler
        FileWrangler
            BuildFileWrangler
            DistributionFileWrangler
            MakerModuleWrangler
            StylesheetWrangler
        PackageWrangler
            MaterialManagerWrangler
            MaterialPackageWrangler
            ScorePackageWrangler
            SegmentPackageWrangler
Renamed session variable.

OLD: Session.is_navigating_to_score_makers

NEW: Session.is_navigating_to_score_maker_modules
Removed session variable.

OLD: Session.current_build_directory_path.
Refactoring Session.

Reimplementing current_* variables in terms of controller stack inspection.

MODEL: the central feature of Session will be the controller stack; almost
everything else can be derived from controller stack inspection.
Extended IOManager._handle_io_manager_directive().

The method now returns 'q' after handling a quit command from the composer.

The general idea is that IOManager._handle_io_manager_directive() should return
a string value in all cases where the IOManager is unable to handle the user
request directly.

The purpose is to facilitate debugging.
Extended IOManager._handle_io_manager_directive().

Now returns 'b' in response to all user back commands.
Cleaning up IOManager._handle_io_manager_directive().

Harmonized variable names.
Cleaning up IOManager._handle_io_manager_directive().

Harmonized more variable names.
Cleaning up IOManager._handle_io_manager_directive().

Method now returns '>', '>>', '<', '<<' as appropriate.
Cleaning up IOManager._handle_io_manager_directive().

Refactored to include only a single return statement.
Cleaned up IOManager.

Removed debugging statements.
Updated TODO.
Removed session variable.

OLD: Session.last_line.

Was unused.
Cleaning up Session.

Removed a couple of variables from the output of (sdv).
Reimplemented menu headers in terms of controller stack instead of breadcrumbs.
Removing breadcrumb management.

Removed two private session methods:

    * Session._cache_breadcrumbs()
    * Session._restore_breadcrumbs()

No longer needed because menu titles are now derived programmatically from the
session controller stack.
Removed comments from previous commit.