Otherwise, we end up propagating those dependencies into targets that
link against that library, which creates unnecessary link-time
dependencies.
Also included are changes to readd now missing dependencies to tools
that actually need them.
This adds player widget with working play/pause controls, a seek bar
which currently only displays the current playback position, and a
button to cycle between the scaling modes.
The player uses the new PlaybackManager class to handle demuxing,
decoding, and frame presentation timing.
Currently, the volume control is non-functional.
