Implement horizontal rule rendering in text

The rule is just a line drawn with the same thickness as an underline:
it gets thicker as DPR increases.

A horizontal rule can be created via <hr/> in RichText.
In markdown it's called a "thematic break," and a line with
- - -
is one way to create it: https://spec.commonmark.org/0.30/#thematic-breaks
<hr width=70/> or <hr width=70%> set the width in pixels, and percent
of the text width, respectively; the rule will be centered within the
line's bounding box in that case, as in QTextEdit etc.

The color can come from QTextFormat::BackgroundBrush if it's set
on the QTextBlockFormat, but otherwise falls back to the text color.
This can be done with CSS styling: <hr style="background-color:green;"/>

[ChangeLog][QtQuick][Text] Horizontal rules (thematic breaks in markdown)
are now rendered as simple horizontal lines, either in the same color
as the text, or as specified via CSS background-color.

Fixes: QTBUG-74342
Task-number: QTBUG-81306
Change-Id: I64f9daf28994225d1a8383d8e2e01e611a0a0237
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

1 files changed, 6 insertions, 0 deletions

diff --git a/examples/quickcontrols2/texteditor/example.md b/examples/quickcontrols2/texteditor/example.md
index 169ff4338e..0ee17cdb9b 100644
--- a/examples/quickcontrols2/texteditor/example.md
+++ b/examples/quickcontrols2/texteditor/example.md
@@ -135,6 +135,12 @@ should end with a period or a parenthesis:
 
 The list will automatically be renumbered during rendering.
 
+## Thematic Breaks
+
+A horizontal rule is possible, as in HTML:
+
+- - -
+
 ## Tables
 
 One of the GitHub extensions is support for tables: