Asynchronous setProperty / command

[moffatman]

This removes all noticeable blocking of dart/ui thread

Some notes/questions for @alexmercerind

1. I replaced the "play() -> get_property -> playOrPause" pattern with just set_property('pause', false). I got bad race conditions with the old way. Not sure why you changed to it originally?
2. I can move seek back to the main thread now :)
3. I didn't change screenshot() since it has one-off use of command-with-result. But also it uses package:image to do some conversion in dart-space, so other Isolate is still a good idea.
4. I didn't implement getProperty. Because the memory ownership there gets a little more complicated. But after rebasing my change here, I see there is now NativePlayer.getProperty. Can think about it some more...

[moffatman]

I guess one more thing to do as well, apply similar changes to media_kit_video/lib/src/video_controller/android_video_controller/real.dart