* Add Widget.border_title and border_subtitle.
Related issues: #1864
* Test setting border_(sub)title.
* Add border (sub)title references to StylesCache.
These internal references will make it easier for the instance of 'StylesCache' to know which border (sub)title to use, if/when needed.
* Add method to render border label.
* Add styles to align border (sub)title.
* Render border labels.
* Update styles template.
* Make new 'render_row' parameters optional.
* Add (sub)title border snapshot tests.
* Document border (sub)title and styles.
* Pass (sub)title directly as arguments.
Get rid of the watchers to make data flow easier to follow.
Related comment: https://github.com/Textualize/textual/pull/2064/files\#r1137746697
* Tweak example.
* Fix render_border_label.
This was wrong because border labels can be composed of multiple segments if they contain multiple styles. Additionally, we want to render a single blank space of padding around the title.
* Ensure we get no label when there's no space.
* Add tests for border label rendering.
* 'render_border_label' now returns iterable of segments.
* Add label to render_row.
* Fix calling signature in tests.
* Add padding to snapshot tests.
* Fix changelog.
* Update snapshot tests.
* Update snapshot tests.
* Border labels expand if there's no corners.
* Update CHANGELOG.md
* Fix docs.
* Remove irrelevant line.
* Fix snapshot tests.
* Don't share Console among tests.
* Simplify example in styles guide.
* Avoid expensive function call when possible.
* rewording
* positive branch first
* remove wasteful indirection
* fix changelog
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Co-authored-by: Will McGugan <willmcgugan@gmail.com>
When defining the formal syntax of CSS rules we will use a syntax similar to that of the MDN docs and we want to be able to link directly from the syntax to the CSS types involved, so we cannot do that from within a markdown code block. Instead, we use explicit HTML that we include in two snippets so that it is easier to maintain.
