* 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>
