This struct is finally going to have to be declared in the header as it
will be needed by wxTreebookXmlHandler when it's modified to derive from
wxBookCtrlXmlHandlerBase too.
Also add GetImageId() function to the struct while moving it.
Add new wxBookCtrlXmlHandlerBase and inherit the XRC handlers of all the
wxBookCtrlBase-derived classes except for wxTreebookXmlHandler, which
will require special handling, from it to avoid duplicating the same
code in all of them.
This commit is best viewed with --color-moved git option.