Implement (hacky) support for scalable icons

[Tharre]

Previously, any icons that were found in folders like scalable or symbolic were ignored because they didn't contain any sizing information. Instead, assume any icons ending with ".svg" to be scalable as it's the only supported vectorized format.

With this, the adwaita icon theme should work again, they removed most of their fixed size icons semirecently:

https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/issues/221