cv::Mat is already reference counted.

Follow interface changes, allow selection of tracking images in main app

PlayerParameters is owned by MediaPlayerStateMachine, which lives on
MediaPlayers thread. sharing a Pointer via signals is not thread safe.
This is fixed by copying the object and using a shared_ptr in the
signal. The shared_ptr is const, so there may only be concurrent reads
on it.

See https://bugzilla.kernel.org/show_bug.cgi?id=199575 as to why just using indices doesn't work

..and slightly less opaque and with a dashed line. This makes it easier to see whether there is an active annotation for the current frame in case of many stacked inactive annotations that would otherwise obscure the active one.

Otherwise, annotations would not be saved when closing the video if the only action since last save was e.g. setting the end timestamp.