Add Lottie marker support to LottieAnimation

Lottie animations can define named markers tied to specific frames.
This exposes those markers through the read-only markers property and
adds gotoAndPlay()/gotoAndStop() overloads that accept marker names.
Markers with a non-zero duration (dr) stop playback automatically at
the end of the segment. Navigation UI is also added to the viewer example.

Task-number: QTBUG-146788
Change-Id: I645ad021ff9e4d9e6bf93506c9e2323d58ea893f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>