Break down the GO_BACK command into separate commands

[Niloth-p]

What does this PR do, and why?

Breaks GO_BACK command into the following commands:

• CLEAR_SEARCH (clears search box and search list of current panel)
• EXIT_COMPOSE (clears compose box state and closes it)
• EXIT_POPUP (closes the currently open popup)

This would enable using more specific help texts, categories and context.
And clarifies the purposes of the key better for users and developers.

I've kept these as separate commits for the sake of readability. Introducing each command in its own commit, then finally removing GO_BACK command.
I could break #1497 into commits similar to this one for readability, if necessary.
I've kept this as a separate PR as it's an independent change, even though the motivation is shared.

External discussion & connections

• Discussed in #zulip-terminal in re-categorize
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR Break down the generalized ENTER command #1497
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

[zulipbot]

Hello @zulip/server-refactoring members, this pull request was labeled with the "area: refactoring" label, so you may want to check it out!

[rsashank]

Since we're aiming at contextual help messages, should we be using the word panel? It feels more like code terminology and might not be useful as a help message.

[Niloth-p]

Hmm.
Found mentions of 'panel' in the user facing documentation.
What other word do you think would fit better in this context?
Edit: As discussed, it's hard to come up with suitable alternatives, so I've left it as 'panel'.

[rsashank]

LGTM! The intention seems clear since we're aiming for contextual help.

Just a note: Either this PR or #1442 will need to handle conflicts in a rebase, depending on which gets merged first.

[zormit]

This is pretty nicely structure and looks good to me. Thank you!

[neiljp]

@Niloth-p I agree with @zormit - this is clear and easy to read 👍

The changes I pushed back with were only structural/text (the code overall is identical):

• squash the last commit with the smaller of the two later commits, to avoid a state where we have a menu entry that does nothing
• adjust some commit text (these change the UI, so aren't strictly refactors
• added some file names in the commit titles