Implement border (sub)title. (#2064)

* 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

---------

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

docs/styles/border_subtitle_align.md

@@ -0,0 +1,62 @@
+# Border-subtitle-align
+
+The `border-subtitle-align` sets the horizontal alignment for the border subtitle.
+
+## Syntax
+
+--8<-- "docs/snippets/syntax_block_start.md"
+border-subtitle-align: <a href="../../css_types/horizontal">&lt;horizontal&gt;</a>;
+--8<-- "docs/snippets/syntax_block_end.md"
+
+The style `border-subtitle-align` takes a [`<horizontal>`](../../css_types/horizontal) that determines where the border subtitle is aligned along the top edge of the border.
+This means that the border corners are always visible.
+
+### Default
+
+The default alignment is `right`.
+
+
+## Examples
+
+### Basic usage
+
+This example shows three labels, each with a different border subtitle alignment:
+
+=== "Output"
+
+    ```{.textual path="docs/examples/styles/border_subtitle_align.py"}
+    ```
+
+=== "border_subtitle_align.py"
+
+    ```py
+    --8<-- "docs/examples/styles/border_subtitle_align.py"
+    ```
+
+=== "border_subtitle_align.css"
+
+    ```sass
+    --8<-- "docs/examples/styles/border_subtitle_align.css"
+    ```
+
+
+### All title and subtitle combinations
+
+--8<-- "docs/snippets/border_sub_title_align_all_example.md"
+
+
+## CSS
+
+```sass
+border-subtitle-align: left;
+border-subtitle-align: center;
+border-subtitle-align: right;
+```
+
+## Python
+
+```py
+widget.styles.border_subtitle_align = "left"
+widget.styles.border_subtitle_align = "center"
+widget.styles.border_subtitle_align = "right"
+```