VideoPlayerWidget was keeping a reference to PlaybackManager when
changing files, so the old and new managers would both send frames to
be presented at the same time, causing it to flicker back and forth
between the two videos. However, PlaybackManager no longer relies on
event bubbling to pass events to its parent. By changing it to send
events directly to an Object, it can avoid being ref counted, so that
it will get destroyed with its containing object and stop sending
events.
No longer will the video player explode with error dialogs that then
lock the user out of closing them.
To avoid issues where the playback state becomes invalid when an error
occurs, I've made all decoder errors pass through the frame queue.
This way, when a video is corrupted, there should be no chance that the
playback state becomes invalid due to setting the state to Corrupted
in the event handler while a presentation event is still pending.
Or at least I think that was what caused some issues I was seeing :^)
This system should be a lot more robust if any future errors need to be
handled.
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.
