[API] Return object paths in GetRenderers() as documented

Funny enough, this was one of the things I noticed and wrote a patch for while trying to work on other things a week ago... just hadn't filed the bug (bad habit, I know).

I think I got all the places we use the signature in, but please double check.

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GetRenderers() should return object paths instead of strings
(documentation already claims it does).

Fixes #110.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>