Message information popup actions (open, copy, ...) vs content (narrows, links, code, ...)

[neiljp]

For a given message, various content is indexed and may be scrolled through when in the message information popup.

Depending upon the message/topic content, this can include one or more of:

• links in the topic (often linkifiers eg. Message information popup actions (open, copy, ...) vs content (narrows, links, code, ...) #1372)
• external links (name + URL)
• media links (~attachments)
• narrow links (eg. #zulip-terminal>some topic)
• code snippets (see Add support for copying code snippets #1123 WIP)

For entries in each section, we likely want to support various actions. Right now we support scrolling amongst all the buttons that could be activated, but only some support an action, and it is not obvious that they do in advance:

• (most) narrow links will go to the indicated narrow
• media links (attachments) will offer to download and open the attachment

Ultimately we may wish

• every button to support 'copy'
• external links and topic links to be opened in a browser
• code snippets to support an 'open in playground'

Particular steps are certainly

• Indicate if activating a focusable entry will do anything, and with what key (eg. [<key>])
• Explore adding copying to each entry, including how to handle multiple actions on an entry
• Add more functionality for each entry where appropriate (check previous PRs)

See previous discussion in #zulip-terminal > Clarifying message actions