This will extract vectorial symbolic icons out of the SVG source, and
generate vectorial symbolic inverted icons too.
Vectorial color icons are not extracted yet.
I also make sure that the tools/ subdir is processed by make before
icons/ because a few build tools will be needed to extract the icons.
Yet I mark the feature as experimental because librsvg seems to be
broken on many edge cases and several icons end up wrong. I'll keep
the option experimental until I figure the right way to extract the
icons.