DispatchTimer is constantly running

[lhecker]

From an internal mail conversation titled "Terminal is always ticking":

I notice that Terminal on my machine is constantly ticking, meaning XAML Tick. It looks like there is a DispatcherTimer constantly running, even when the app isn't in the foreground and even when it is minimized.

We should stop any DispatchTimers when Terminal is minimized.

[zadjii-msft]

May also want to refer to #1989 while we're at it. Conhost (I believe) stopped rendering when minimized, but I'm not sure Terminal is hooked up to know [A] when the window is minimized, and [B] when a TermControl is or is not part of the visual tree.