HTML tag support is aligned with Mastodon 4.2 supported tags.
This code introduces a soft dependency on the urwidgets library.
If urwidgets is not available, HTML tags are still supported,
but hyperlinks are not underlined using the OCS 8 terminal
feature (on supported terminals).
Monochrome colors should be defined as the third color in each tuple
instead.
In monochrome mode it's not possible to set the background so all
highlights can only be done using bold, italics, underline...
Some terminal color schemes completely eliminate the difference
between cyan and cyan-bold colors (all the base16 themes, for instance).
This change makes the key letters stand out clearly in bold white.
Reorder Visibility Options to be from "most visible" to "least visible",
and swap the definitions of "private" and "unlisted" to match the actual
visibility that results from those actions.
