Because Textual uses on_ for event handlers there was the danger of a name
clash; so to keep things as clear as possible this renames anything to do
with "on" (method names, properties, style classes) so that it talks about
"filled" instead.
See https://github.com/Textualize/textual/pull/963#discussion_r1000544563
It had been suggested to me that these would be needed, but in testing here
I'm not seeing that. So, until I find out otherwise, let's simplify things
and drop that.
Originally I was doing everything in the DOM, using just the primitive
widgets. Given that I recently created an actual GameCell widget (which
simply inherits from a Button, but still...) it makes sense to now have
row/col properties as part of that.
