Add border-text-style, border-color, and border-background styles

[willmcgugan]

Add the following styles:

title-color
title-background
title-text-style

subtitle-color
subtitle-background
subtitle-text-style

These should have the same semantics as color, background, and text-style (should be able to re-use some code here).

The combined styles should apply to the border title subtitle.

Note that in the border there is a space either side of the title / subtitle. This space should have the same style as the text.

[willmcgugan]

One of the reasons for this is that we can create borders like the following:

This would look great if the title and space around it could be flipped in color.

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.