Try to reduce freezes in the ScreenViewControl

* Insert an additional toplevel hosting window into the chain
* Increase resize timeout and hide magnifier while resizing
* Hide magnifier window when it can't be seen
* Correct calling convention in the declaration of magnifier functions

On the test VM, this has resulted in almost no random freezes,
but increased VirtualBox crashes. We'll see how it turns out on the
target machine.
Darken color Scheme and minimize window padding

I'm not convinced yet, but the target machine's screen
size is so small, every pixel counts.