Second batch of changes to the tab-layout for inclusion into McCLIM:

    These changes move all STACK-LAYOUT and RADIO-LAYOUT mechanisms
    directly into the TAB-LAYOUT.  While the former two may still be
    useful to someone, the resulting TAB-LAYOUT does not depend on them
    anymore.

    This new codebase is perhaps less modular than the original layered
    version.

    However, it fixes a conceptual bug of the original design, namely
    that the child panes of the TAB-LAYOUT-PANE were not actual, direct
    children of it.  Instead there was the RADIO-LAYOUT sitting between
    TAB-LAYOUT-PANE and pages.

    This made the operation "return the tab-layout this pane is a page
    of" needlessly complicated.  In the new implementation, this
    operation is just SHEET-PARENT.

    The new design is also what Gtkairo would have to implement in its
    subclass anyway, so now we get the chance to move all logic
    concerning the sheet hierarchy directly into the abstract superclass
    instead of reimplementing it in the subclasses.


Other changes:

  - Added a setf method to TAB-LAYOUT-PANES which allows panes to be
    added, removed, and reordered using the same function.  ADD-PAGE is
    still available as a convenience wrapper..

  - Removed REMOVE-PAGE-USING-PARENT again, since SHEET-DISOWN-CHILD now
    serves the same purpose.  User code can still use REMOVE-PAGE.
    Also added a generic function INTERNAL-CHILD-P that TAB-LAYOUT
    subclasses for other frame managers might want to implement.

  - Moved the slot backing TAB-LAYOUT-ENABLED-PAGE into the superclass,
    which now maintains it.

  - Improvement: When the current page is removed, drop to the page next
    to it, if any.  Previously focus would always jump to the last page
    (at the end).

  - Changed the default presentation type to be TAB-PAGE.

  - Allow arbitrary initargs to be passed through WITH-TAB-LAYOUT.

  - Added a generic function note-tab-page-changed for subclasses to
    implement, which informs the layout about changes to tab-page-title
    and -drawing-options.

  - Reworked the tab-layout-example for inclusion into McCLIM.