Play/pause toggle button uses pause icon

The button is always a media-playback-pause-symbolic regradless of what it will do when pressed. It's a bit confusing when I see this UI for the first time.

The tooltip shows playback info, so there's no way to know if the button is a toggle without clicking it.

I think there're two ways to solve this:

1. Change it to use the media-playback-play-pause-symbolic icon to make it clear that the button is a toggle.
2. Show media-playback-play-symbolic or media-playback-pause-symbolic icon according to the current status.