Improve frame rate switching

[mseeley]

We should change Refresh Rate Switching such that:

• We switch framerate when a new video starts
• We switch back when the video player is closed

Because right now, framerate is switched as soon as the postplay screen appears. This can mean on slower TVs, that the countdown clock is almost run out by the time the display has finished switching, then if it's the same TV show that starts, it probably needs to immediately switch back to the framerate it was playing in anyway.

By taking control of the framerate switches, we can keep the postplay screen in the same framerate as the TV show, making watching multiple episodes far less irritating, and only switch back to the system framerate when the postplay screen is closed.

[mseeley]

This is tightly related to #754. Same too eager display-rate switching.