Encounter Muncher: Provide more scene options

[MrPrimate]

For encounter muncher, it'd be nice to offer a template for the scene beyond just a background image (background color, grid area size, etc). A user could construct one (multiple?) json file (or export it from a tool) which you could read from (select from the existing background selection drop menu), as one idea. Also, for the existing background image option, could you scale the image to fit the entire grid area instead of just a small section of it? Either that or shrink the grid area to fit the image.