Add show color demo.

This will let you explore color names or color codes as they display
in your terminal.

fixes #769 UnderlineStyle is displayed, but UnderlineColor is not

Bump golang.org/x/text to 0.21.0

windows: restore cursor location correctly

terminfo: regenerate all entries

Previously we had lost StrikeThrough, and this fixes a few other
discrepancies.  This was generated on Ubuntu Noble (24.04), after
installing the following packages: ncurses-term kitty-terminfo

Try not to clear screen on Windows if TCELL_ALTSCREEN=disable

fixes #621 Unicode disappears if requested twice on the same place on…

… Windows

fixes #606 Want ColorNone to preserve existing color(s)

While here, consolidate the use of the Fill() function from
CellBuffer (eliminating redundant code).

minor style fixup

Fix status badges (broken by shields.io)