Sketch text: bold and italic, text along a curve, alignment and spacing (#852)

A sketch text gets an optional style: bold and italic faces (a slanted fallback for italic), left/center/right alignment, letter and line spacing, and a path curve of the same sketch for the baseline to follow (reverse takes the other side). Edit Text gets B/I toggles, alignment, spacing fields and a Path chip, all live. The style is saved only when not plain and is noted under format 18.

crates/engine/src/view_display_tests.rs

@@ -25,7 +25,7 @@ fn sampler_keeps_style_precedence_hidden_references_text_and_disjoint_parts() {
     let text = sk
         .add_link(
             LinkKind::Text,
-            LinkSource::Text { text: "A".into(), at: Vec2::ZERO, height: 2., angle: 0., font: None },
+            LinkSource::Text { text: "A".into(), at: Vec2::ZERO, height: 2., angle: 0., font: None, style: Default::default() },
             &[LinkGeom::Line(Vec2::new(0., 7.), Vec2::new(2., 7.))],
         )
         .unwrap();