Icon aliases cleanup

[Martin Měřinský]

> looking as our icon set (the ones in subsurface/icons/) we seem to
> have a repeated one: warning.svg and warning-icon.svg.

Good observation. I know, and have more patches prepared, that will
clean this up.

> Also (and probably there's an obvious reason for it, that I don't
> know), why do we keep svg and png versions of the same icons?

SVG icons are usualy "source". They can be used directly for several
icon sizes. Or, if needed, PNG icons are generated from SVG and fine-
tuned for particular size. For future changes, we want to keep the
source, so we keep both SVG and PNG (but in better directory
structure).

Cheers Martin M.