Initial tab-layout modifications for inclusion into McCLIM.

  - temporarily added a common system definition clim-tab-layout.lisp
    (to be removed once the tab-layout moves into McCLIM proper)

  - temporarily added a common package file package.lisp
    (to be moved into mcclim/package.lisp ultimately)

  - temporarily moved stack-layout and radio-layout into the same package
    (ultimately, the tab-layout will not be radio-layout-based at all anymore)

Changes to the stack- and radio-layout:

  - removed the hand-written ENABLED-PANE wrapper for the slot
    ENABLED-RADIO-PANE in favour of a direct slot reader
    RADIO-LAYOUT-ENABLED-CHILD.

  - Add a setf method (SETF RADIO-LAYOUT-ENABLED-CHILD) as the primary
    interface instead of SWITCH-TO-PANE.

  - Keep SWITCH-TO-PANE as a convenience wrapper, but remove its second
    argument.

  - In INITIALIZE-INSTANCE, support having no children initially.

  - Replace ADD-PANE and REMOVE-PANE and their methods with methods
    defined directly on SHEET-ADOPT-CHILD and SHEET-DISOWN-CHILD so that
    user code and use normal CLIM functions to modify the sheet hierarchy.

Changes to the tab-layout:

  - For frame manager support, split the TAB-LAYOUT-PANE into the
    abstract superclass TAB-LAYOUT and its generic TAB-LAYOUT-PANE
    subclass.

  - Rename TAB-PANE to TAB-PAGE, since it's -not- a pane.
    The presentation type is also called TAB-PAGE, commands changed
    accordingly.

  - Rename TAB-PANES-OF-TAB-LAYOUT to TAB-LAYOUT-PAGES.
    Rename TAB-PANE-TITLE to TAB-PAGE-TITLE and export the latter.
    Rename TAB-PANE-PANE to TAB-PAGE-PANE and export the latter.
    Rename TAB-PANE-PTYPE to TAB-PAGE-PRESENTATION-TYPE and export the latter.
    Rename DRAWING-OPTIONS-OF-TAB-PANE to TAB-PAGE-DRAWING-OPTIONS
    and export the latter.

  - Add :DRAWING-OPTIONS as an initarg to TAB-PAGE-DRAWING-OPTIONS slot.

  - Add a new slot TAB-PAGE-TAB-LAYOUT, automatically maintianed when a
    page is added to and removed from the tab-layout.

  - Added a new slot TAB-PAGE-ENABLED-CALLBACK, fixing a terrible kludge
    in beirc, which wants to be notified when a page is enabled.
 
  - New function FIND-TAB-PAGE-NAMED, replaces the STRING case of
    FIND-IN-TAB-PANES-LIST.

  - New function SHEET-TO-PAGE, replaces the SHEET case of
    FIND-IN-TAB-PANES-LIST.

  - Rename ENABLED-PANE to TAB-LAYOUT-ENABLED-PAGE.

  - Rename SWITCH-TO-PANE to SWITCH-TO-PAGE.   Remove its second
    argument.

  - Rename MAKE-TAB-PANE-FROM-LIST to MAKE-TAB-PANE, since there is no
    list as an argument.  This is an internal helper function for our macro.
    User code is encouraged to just use MAKE-INSTANCE instead.

  - Removed MAKE-LIST-FROM-TAB-PANE, since it was not being used.

  - Remove SET-DRAWING-OPTIONS-FOR-PANE-IN-TAB-LAYOUT in favour of the
    normal slot accessor TAB-PAGE-DRAWING-OPTIONS.

  - Replace ADD-PANE with ADD-PAGE, split into general methods for the
    superclass and subclass.  Add some error checking code.

  - Replace REMOVE-PANE with REMOVE-PANE-USING-PARENT.
    Add a new function REMOVE-PAGE as a convenience one-argument
    version.  Add some error checking code.

  - Add a convenience function REMOVE-PAGE-NAMED, replacing the STRING
    method of REMOVE-PANE.

  - No replacement is offered for the SHEET method of REMOVE-PANE, users
    should call (remove-page (sheet-to-page <sheet>))  instead.

  - Change WITH-TAB-LAYOUT to create the abstract TAB-LAYOUT, moving all
    implementation-specific code into new INITIALIZE-INSTANCE methods on
    TAB-LAYOUT-PANE.

    Also fixes the macro to not evaluate the panes multiple times.

    Change the syntax as parsed by MAKE-TAB-PAGE to accepts arbitrary
    keyword arguments.

    Application code that use the optional ptype argument need to switch
    to the :PRESENTATION-TYPE keyword.  New keyword arguments are
    :DRAWING-OPTIONS and :ENABLED-CALLBACK.