Text truncation with ellipsis

[valadaptive]

See the CSS text-overflow property.

If we want to truncate some text rather than wrap it, it'd be nice to do that cleanly with an ellipsis instead of just clipping it:

egui supports arbitrary text truncation strings (Firefox also extends text-overflow to support that too); I'm not sure how difficult that would be.

It seems the appearance (font and size) of the ellipsis is determined in CSS by the "parent" style, which requires us to define that as well--see https://github.com/linebender/parley/issues/291.

This is necessary for porting egui to Parley.