Single entries improvements

[thupsi]

Pending some kind of "Unified Content View" or similar, and partly due to the ongoing "entrification", the display of singles in the CP would benefit from some UI enhancements.

1. Singles are displayed to editors like ephemeral "entries", with the details pane on the right etc, even when a single is meant to be a container for global settings (former Global Set). So, similar to 17642, but going in the oposite direction, I would like to have a setting to hide the details pane entirely for specific singles (or even for any section/entry type, why not?). This would make for a cleaner UI where needed.
2. Singles require two clicks to open: First click to view the single in the index, then click the index entry to open, even when each single is moved to its own custom source, so there's no point of having an index displayed at all. And even if the singles are set to open on click with the "Expanded Singles" plugin, the side sources list disappearing makes for a disorienting experience. To this end, I propose to bring us the option to "ungroup" singles, open each single when clicking its name on the sources list, and keep the sources list visible when editing the single. If that sounds confusing, it's the same pattern used for Global Sets.

In reality both of the above suggestions would enable us to emulate a Global Set's UI, but use a Single instead.

To illustrate:

The current UI with a single that is supposed to contain some settings.

Problems:

1. I have to click "Rates" and then again "Rates" to get to it.
2. Even having disabled the "slug" and "status" fields I still have a kinda useless "details" pane on the right.
3. The breadcrumbs look weird, with "Rates" repeated two times.
4. If I want to move laterally to another source, tough luck.

Moving on:

Proposed UI improvements. Note that these are entry sources, not global sets.

Enhancements

1. I click "Rates" and the edit form opens. Just one click.
2. The unneeded details pane is hidden via the proposed setting.
3. The breadcrumbs issue is solved.
4. I can move laterally between sections/sources without being disoriented.
5. Even with the sources list visible, there's more room for the edit form, because the details are disabled.

Notes:

1. If the details pane is not disabled, having the "sources list" visible like the above screenshot would result in the edit form possibly being too narrow. I suggest that in that case (if the details pane is not disabled), the sources list should only remain visible on wide enough screens. That seems to me like a reasonable compromise.
2. It goes without saying that If we are in the future able to nest sections under singles, like suggested in 17449, it would make sense to only use the "open single on source name click" pattern when the source contains just one single.

I suspect that the team has plans to tackle some of these issues in Craft 6, but I guess there's no harm putting my ideas in writing.

Let me know what you think!