Add simple wxBitmapBundle::FromSVG() implementation using NanoSVG

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).

build/cmake/files.cmake

@@ -919,6 +919,7 @@ set(GUI_CMN_SRC
     src/generic/rowheightcache.cpp
     src/generic/animateg.cpp
     src/common/bmpbndl.cpp
+    src/generic/bmpsvg.cpp
 )
 
 set(GUI_CMN_HDR