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Provides a base class for folderish content types based on B-trees

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This package provides a base class for folderish content types based
on `B-trees`_, a.k.a. "large folders" in Plone_.  Storing content in
such folders provides significant `performance benefits`_ over regular
folders.  However, "large folders" do not support explicit ordering of
their contents out-of-the box.  That is, you cannot manually specify the
order of items within the folder, you can only sort things according to
a given criteria after fetching items from the folder.

  .. _`B-tree`:
  .. _`B-trees`:
  .. _`Plone`:
  .. _`performance benefits`:
  .. |---| unicode:: U+2014  .. em dash

Many times, though, it is desirable to be able to explicitly order
a folder's content, for example for items that are related to site
navigation.  Sorting alphabetically often doesn't make sense here.

To compensate ``plone.folder`` provides ordering support for `B-tree`_
folders via the above mentioned base class, which can make use of an
adapter to store the actual order information.  It also comes with two
sample adapter implementations:

  * A default adapter handling order information for all objects
    contained in a folder.  This adapter can be used to build fully
    backwards-compatible drop-in replacements for folderish content.
    In other words, when using this adapter `B-tree`_-based folders should
    behave just like the "regular" folder implementation, but provide some
    of the performance benefits at the same time.

  * An alternative adapter implementation that is targeted towards
    sites with only a relatively small percentage of content for which
    (explicit) order matters.  It'll only manage order information for
    certain content types (via a marker interface).

The latter allows to not having to separate such content from
"non-orderable" content.  Up to now many large sites ended up storing
"orderable" items |---| for example everything related to navigation
and typically only a few |---| in regular folders and the bulk of the
content in "large" folders, most of the time solely for performance
reasons.  This adapter will hopefully help avoid having to make this
distinction in the future and still provide the better performance
characteristics of `B-tree`_ folders.
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