Add marker navigation support to LottieVectorImageController

Lottie animations can define named markers tied to specific frames.
VectorImage now exposes them, and LottieVectorImageController adds
gotoAndPlay()/gotoAndStop() overloads that accept marker names.
Markers with a non-zero duration stop playback automatically at the
end of the segment. Navigation UI is also added to the viewer example.

Task-number: QTBUG-146990
Change-Id: I35aca483af6f757338d62ecd7676ca97c179ffab
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>