Rename "on" things to "filled" things

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

examples/five_by_five.css

@@ -47,12 +47,12 @@ GameCell:hover {
     border: round $panel;
 }
 
-GameCell.on {
+GameCell.filled {
     background: $secondary;
     border: round $secondary-darken-1;
 }
 
-GameCell.on:hover {
+GameCell.filled:hover {
     background: $secondary-lighten-1;
     border: round $secondary;
 }