feat: a sketch text's height, place and angle can be expressions (#1453)

A sketch text's height, place (x and y) and angle can each be an expression over the design's parameters, and the text follows parameter changes, undo and reopening. Closes #1362.

Co-authored-by: NOWN-DEV <[email protected]>

docs/book/src/guide/files.md

@@ -75,7 +75,7 @@ is available. Missing or incompatible pictures use the usual thumbnail generatio
 The default `cache: false` omits the portable thumbnail too.
 The repository's `docs/design-container.md` describes limits and encoding details.
 
-Current designs use format 21, including design properties, named render setups, opaque PBR looks, thin extrudes, sheet metal perforation fields, associative editable sketch patterns, editable associative offset-distance dimensions, the style of sketch text (bold, italic, alignment, spacing and the curve it follows), face anchors on a piece of a face by its own name, sheet metal rims and bend ends, bend edges for Convert to Sheet Metal, the tear corner relief of sheet metal rules, the names of projected edges, physical materials defined in the design, the font of sketch text, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
+Current designs use format 21, including design properties, named render setups, opaque PBR looks, thin extrudes, sheet metal perforation fields, associative editable sketch patterns, editable associative offset-distance dimensions, the style of sketch text (bold, italic, alignment, spacing and the curve it follows), face anchors on a piece of a face by its own name, sheet metal rims and bend ends, bend edges for Convert to Sheet Metal, the tear corner relief of sheet metal rules, the names of projected edges, physical materials defined in the design, the font of sketch text, the expressions behind a sketch text's height, place and angle, sheet metal corner relief shapes, sketch face anchors on cylinder caps, sketch axis
 links, persistent
 names from exact face history, exact planar shell-face anchors, persistent analytic offset boundary roles and
 association profile references, durable sketch face anchors, periodic fit splines and versioned

docs/book/src/guide/sketching.md

@@ -47,16 +47,24 @@ circle and update the outline when the inline **Sides** value changes.
 `sketch.edit_text` take a `font`, the family or full name of an installed font ("DejaVu Sans") or the path
 of a `.ttf` / `.otf` file, and the **Edit Text** dialog has a **Font** field for it. An unknown font is an
 error, and the font is kept with the text in the design, so the text stays editable in it. `SOLIDCRAFT_FONT_DIRS`
-adds folders to the ones searched for installed fonts. The same two commands refuse a `text` that is not a
-string, an `at` that is not two numbers, and a `height` or `angle` that is not a number (or a `height` that is not
-positive) with a message, instead of drawing the text with a default; left out, they keep their defaults.
+adds folders to the ones searched for installed fonts. A text's `height`, its `angle` and each coordinate of
+its `at` can be expressions of the design's parameters, given as strings (`"height": "plate_height / 10"`,
+`"at": ["plate_width / 2", 5]`): the text keeps the expression and follows the parameters when they change, and
+saves it with the design. A number takes the place of an expression, and so does moving the text or dragging its
+handles (what moves becomes a number; turning a text leaves its height as it was). A string that names no
+parameter is the number it comes to. The same two commands refuse a `text` that is not a string, an `at` that is
+not two numbers or expressions, a `height` or `angle` that is neither (or a `height` that is not positive) and an
+expression that cannot be worked out with a message, instead of drawing the text with a default; left out, they
+keep their defaults.
 
 Right-click a letter's filled interior or outline and choose **Edit Text**. You can also
 start **Edit Text** first and click the letter. All outlines belonging to that text are
 highlighted while the dialog is open. A profile bounded by multiple text sources or other
 geometry cannot identify one text: use **Select Other** to choose its outline instead.
 Change the text, font, baseline X/Y, height or angle, then confirm with **OK**; **Cancel**
-keeps the original text. Position and height use the document's units.
+keeps the original text. Position and height use the document's units. The four fields take
+expressions: one that names a parameter (`plate_width / 2`) stays an expression, is shown as typed
+when the dialog is opened again, and keeps the text following the parameter.
 
 A text can follow a curve of its sketch (its **Path**). That curve becomes construction: the
 letters stand on it and their descenders cross it, so as a normal curve it would cut them into