go.*,text,widget{,/material}: implement text truncators

This commit adds support for the idea of a text "Truncator", a string
that is shown at the end of truncated text to indicate that it has been
shortened because it would not fit within the requested number of lines.

When specifying a maximum number of lines, a truncator symbol is always
used. If the user does not provide one, the rune `…` is used. This
requirement results in a better user experience and significantly simpler
code, as we can rely upon the presence of one or more truncator glyphs in
the output glyph stream when truncation has occurred.

When interacting with truncated text, the truncator glyphs all act as
a single, indivisible unit. They can be selected or not, and if selected
they act as the entire contents of the truncated portion of the text.
This means that copying all of a truncated label will copy the entire
label text content, with the truncator symbol not appearing at all.

Concretely, the exposed text API now accepts a Truncator string in
text.Parameters, and there is a new glyph flag FlagTruncator which indicates
that the glyph is part of the truncator run. The truncator run will only
have a single FlagClusterBreak (even if the run would usually have many),
and the glyph with both FlagClusterBreak and FlagTruncator will have the
quantity of truncated runes in its Runes field. This necessitated increasing
the size of the Runes field from a byte to an int, as it's theoretically possible
for quite a lot of text to be truncated.

This commit necessarily bumps our go-text/typesetting dependency to the version
exposing truncation in the exported API.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/text.go

@@ -50,6 +50,9 @@ type textView struct {
 	SingleLine bool
 	// MaxLines limits the shaped text to a specific quantity of shaped lines.
 	MaxLines int
+	// Truncator is the text that will be shown at the end of the final
+	// line if MaxLines is exceeded. Defaults to "…" if empty.
+	Truncator string
 	// Mask replaces the visual display of each rune in the contents with the given rune.
 	// Newline characters are not masked. When non-zero, the unmasked contents
 	// are accessed by Len, Text, and SetText.
@@ -459,6 +462,7 @@ func (e *textView) layoutText(lt *text.Shaper) {
 			PxPerEm:   e.textSize,
 			Alignment: e.Alignment,
 			MaxLines:  e.MaxLines,
+			Truncator: e.Truncator,
 		}, e.minWidth, e.maxWidth, e.locale, r)
 		for glyph, ok := it.processGlyph(lt.NextGlyph()); ok; glyph, ok = it.processGlyph(lt.NextGlyph()) {
 			e.index.Glyph(glyph)