Respect icon theme when loading icons

[kchibisov]

I'm not entirely sure how it all works, but at least there should be a way to point to something else than hicolor.

[chrisduerr]

The current implementation certainly goes more in the "MVP" direction, one could consider just using a crate if a more sophisticated icon lookup is desired. Though IIRC the crates available performed poorly in my tests (but they do have much more complete implementations).