API for adding notebook pages from plugins added (platforms build breakage)

Hello,

I have just committed a patch that adds API so plugins can add
notebook pages to dialogs that support it (the Preferences dialog is
currently the only one). This was requested by uwog for the new collab
stuff.

The cornerstones are:
+ AbiNativeWidget typedef that is the platform's native widget (e.g.
GtkWidget on unix).
+ XAP_NotebookDialog interface that needs to be implemented by dialogs
that support plugin provided pages (look at AP_UnixDialog_Options for
reference).
+ XAP_DialogFactory::registerNotebookPage() and
XAP_DialogFactory::unregisterNotebookPage() for the plugins to
hook/unhook.

The attached patch against abiword-plugins shows a working example by
providing a dummy preferences page for the abigoffice plugin (choice
of abigoffice because it uses gtk already so the patch is
illustrative).

What needs to be implemented for win32/cocoa/...
+ provide AbiNativeWidget in ut_misc.h
+ fill XAP_DialogFactory::_dlg_table::m_tabbed in
ap_Win32Dialog_All.h/ap_CocoaDialog_All.h (look at
ap_UnixDialog_All.h)
+ adapt AP_@PLATFORM@Dialog_Options, when that's done we'll move the
interface derivation from unix to xp for the options dialog.

Best,
Rob