Fix set IsPlaying property of MediaElementBase after call Pause() method

[alex-buraykin]

Fix #56

[xmedeko]

I am not the original author of this code, so I do not know the intent why it has been made this way. But I guess that SetIsPlaying(true) is called when the video really plays. SetIsPlaying(false) is called just when there's an intent to stop the video.

E.g. if you have a button <Button Content="Pause" Enabled="{Binding IsPlaying, ElementName=player", then it is disabled as soon as the Pause() is called - the button is pressed. After your change, it may be disabled a moment after the button is pressed, so the user may press it twice.

What's your use case - reason for this change?