Add nanosvg submodule and use it in the generic implementation of this
function.
This is incomplete yet and, notably, doesn't cache the rasterized
images, but already shows that using SVG images works (at least in the
toolbar sample).
This first version provides only a generic implementation of
wxBitmapBundle using a collection of bitmaps and only supports using it
in wxToolBar in wxMSW and wxGTK.
More methods of creating wxBitmapBundle will be provided later and more
functions taking wxBitmap will be changed to take wxBitmapBundle later
too and, of course, all the other ports will be updated to use the new
API too.
