Tabbed content and border titles

See blog post...

https://textual.textualize.io/blog/2023/03/22/textual-0160-adds-tabbedcontent-and-border-titles/

[0.16.0] - 2023-03-22

Added

• Added parser_factory argument to Markdown and MarkdownViewer constructors #2075
• Added HorizontalScroll #1957
• Added Center #1957
• Added Middle #1957
• Added VerticalScroll (mimicking the old behaviour of Vertical) #1957
• Added Widget.border_title and Widget.border_subtitle to set border (sub)title for a widget #1864
• Added CSS styles border_title_align and border_subtitle_align.
• Added TabbedContent widget #2059
• Added get_child_by_type method to widgets / app #2059
• Added Widget.render_str method #2059
• Added TEXTUAL_DRIVER environment variable

Changed

• Dropped "loading-indicator--dot" component style from LoadingIndicator #2050
• Tabs widget now sends Tabs.Cleared when there is no active tab.
• Breaking change: changed default behaviour of Vertical (see VerticalScroll) #1957
• The default overflow style for Horizontal was changed to hidden hidden #1957
• DirectoryTree also accepts pathlib.Path objects as the path to list #1438

Removed

• Removed sender attribute from messages. It's now just private (_sender). #2071

Fixed

• Fixed borders not rendering correctly. #2074
• Fix for error when removing nodes. #2079