Add background image

[fsobolev]

A new page in preferences window will be added where it will be possible to upload an image, set its scale relative to window size (position will always be centered), and optionally bind image scaling factor to one of bars to get an effect similar to the one shown here.
It will be possible to select multiple images and switch between them similar to color profiles.
An image will always be rendered behind bars.

[gregorni]

I think the image will give more of an emphasis on the rhythm, which is always good to have. Also, it makes the app a bit more personal. A good idea!