Add configuration for wallboard sizing

It is difficult to get Wallboards looking even good (let alone great).

One problem is the automatic sizing of the Slideshow presentation, which is not adjustable via browser zoom.

It is currently very tedious to experiment repeatedly with various combinations of layouts and widget placement to try to get everything actually visible on the screen, yet still big enough to be readable.

We are using a 16:9 screen (which most modern screens are). Perhaps the wallboards were designed for and work better for the older standard ratio 4:3.

-Johnny