Add the ability to shape Waffles as a rectangle, not only as a square or dot

[Blindstars]

Is your feature request related to a problem? Please describe.
The Waffle widget only has a dim parameter which forces dotty or square WafflePixels.

Describe the solution you'd like
Have to ability to size a Waffle as a rectangle in both X and Y dimensions. I have done something similar in one of my own projects:

Describe alternatives you've considered
I have done this in one of my own projects by overriding a couple methods from Waffle and WafflePixel. We could instead add a parameter to the Waffle for the other dimension and refactor WafflePixel to handle both dimensions parameters. dotty would be unaffected by this since it would only use the existing dim parameter.

Additional context
I can propose a PR implementing this feature if this is of interest for the team. Still need to figure out proper parameters implementation for dim_x and dim_y without breaking backwards compatibility. I am guessing we should conserve dim and only add dim_y.