Add a preview_layer property

[waveform80]

@Dishwishy has done something interesting with their fork of picamera: Dishwishy/picamera@dde849a. It seems as if it's necessary to lower the "layer" of the preview (an attribute I don't know anything about yet) to render things on top of it. Given this is an eventual goal of #16 we should add a preview_layer property to permit configuration at runtime.

[Dishwishy]

I'm not sure what it does either - I had to tinker with it to help fix an issue where I wanted a countdown timer run on top...the timer is a total hack of some of the OpenGL examples that shipped with raspian. Here's what I changed in the hello_font project - also a layer attribute.
https://github.com/Dishwishy/pi_booth/blob/master/hello_font/main.c#L96