Make the read receipt marker optional

[FrostyX]

Created from PR #136

The premise is that I have my chat windows on a separate monitor to read new messages without moving the cursor to the actual window. As a consequence, the read receipt markers are inaccurate and not helpful.

In the PR, we discussed that:

• We cannot disable the read receipt markers completely - otherwise, other clients won't know where we left off reading
• We don't want to hide the marker by adjusting its background and foreground colors - because then j, k in Evil mode step on this invisible line

[alphapapa]

Thanks.

• We don't want to hide the marker by adjusting its background and foreground colors - because then j, k in Evil mode step on this invisible line

What if j/k were bound to move between events here? https://github.com/emacs-evil/evil-collection/pull/734

[Stebalien]

What if j/k were bound to move between events here? https://github.com/emacs-evil/evil-collection/pull/734

🙅 That would break keyboard navigation/selection.

IMO, a better approach would be:

1. Mark it as intangible.
2. Automatically enable cursor-intangible-mode in the buffer.

[alphapapa]

@Stebalien That's an interesting idea, thanks. I'll try it out.