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

GetRenderers() should return object paths instead of strings
(documentation already claims it does).

Fixes #110.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
[API] Use object paths in LostRenderer and FoundRenderer

LostRenderer and FoundRenderer signals should have object path
output parameters (instead of string). Documentations refers to
paths already.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>