IconEngine: Forward API to child icon engine

[palinek]

..instead of doing own implementation.

[tsujan]

I'd experimented with almost exactly the same thing myself but I didn't make a PR for 2 reasons:

1. Except for xdndworkaround.cpp, we never used a Qt private header anywhere. I really prefer to keep it that way; otherwise, it wouldn't be worth the probable future hassles.
2. What follows was very confusing to me and I didn't have time to investigate: it worked with my icon set (https://github.com/tsujan/Plataro) but not with the Breeze set. My guess was that either Breeze had a problem or the problem wasn't completely in Fm::IconEngine.

Since the patches are practically identical, I'm sure I'd see the same things with this too.

[tsujan]

Oh, right now, I investigated 2 :) Yes, the Breeze set only includes actions/24@2x in its index.theme, while it does have places/24@2x inside its directory tree. So, 2 is Breeze's fault and I was wrong at #788 (comment) saying, "Breeze is better for testing".

1 is still valid and worries me. But yes, this works.