We don't need to commit PNG symbolic icons as `gtk-encode-symbolic-svg`
is perfectly able to generate them from the SVG.
Moreover its man says that it makes "specially prepared png files",
whatever that means. So I don't think we should just rename our
self-exported PNG files as '*-symbolic.symbolic.png'. It is better to
generate them with this tool.

Note that I am now splitting icon-list.mk into 2 files. This is only a
temporary stepback, as I am actually going to merge the 2 icon themes
into one very soon, and hopefully we should be able to generate the list
to ensure we don't miss an icon.
Anyway the icon lists had already diverged earlier, even though it was
only all still in a single file.