Rename prompter concepts

[aadcg]

While working on #2555, I noticed that prompter's concepts can be difficult to internalize.

I suggest the following rename (reasoning below):

• selection -> current-suggestion
• selection-actions -> current-suggestion-actions
• marks -> selection
• marks-actions -> selection-actions

The key observation is that marks only exist when multi-selection-p is non-nil, which immediately makes us think of a selection (a set of items). But that collides with our concept for "the current suggestion", which we call it selection...

In #2527 I have proposed marks to be a singular concept, and selection falls in that category too. This proposal makes #2527 obsolete.

With this rename the overview of the prompter library would be as follows.

The key objects are =prompters=, =sources= and =suggestions=.

A =prompt= is an interface for user interactions that holds one or more
=sources=, and each of those are populated by =suggestions=.

Other central concepts include:

• =prompt=
  • =current-suggestion= :: A single =suggestion=, if any. Think of it as the currently
    selected =suggestion=, belonging to a =source=.
• =source=
  • =selection= :: A list of =suggestions=, when =multi-selection-p= is non-nil.
  • =actions= :: A list of functions that run on =suggestions=.
    • =return-actions= :: General purpose.
    • =selection-actions= :: On =selection= change (event-driven).
    • =current-suggestion-actions= :: On =current-suggestion= change (event-driven).

[aartaka]

But then marks-actions should be selection-actions, no? Which collides with the existing ones...

[aartaka]

Other than that, selection and current-suggestion sound good to me, yes!

[aadcg]

Oops, good point @aartaka. I forgot to update the action-related concepts. I've edited the top post, please take a look.

current-suggestion-actions is perhaps a bit verbose, but I'm afraid we can't do much better...

[aartaka]

Well, there can be other shorter terms for current suggestion. Like cursor, focus, point, locus?

[aadcg]

The issue is that prompter is a just a backend libary, so I'm afraid those suggestions might not be suitable.

[aartaka]

I mean, cursor is a data structure term as much as UI one, and focus is merely a generic word for one's center of attention at a certain moment in time. Those two are kind of suitable, even for a backend library.

[aadcg]

Fair.

Let's wait for @Ambrevar's input.

[Ambrevar]

Well, selection is a pretty good term for the current suggestion :)
Can also have selections and current-selection.

Let's sleep over this.

[aadcg]

@Ambrevar I'd like to ping on this issue since it's extremely important.

The prompt buffer entry on the manual is perhaps the most important one, since this special buffer is the main interface from the user's perspective.

In other to write a proper entry, we need to settle on a set consistent concepts between prompter (the external library) and prompter-buffer (in nyxt's package).

When we reach consensus, let's incorporate this into #2630.

[Ambrevar]

I'm fine with the suggestion from the original post.

[aadcg]

Didn't add this bit as part of #2630 so that it would be easier to review. Will open a separate PR on that.