[Feature Request] Show heading-folding toggles always

[davemacdo]

Issue

I find it frustrating that I can open a collapsed heading by clicking the "folded" symbol in the left-hand gutter, but that the toggle disappears and I can't recollapse it after. I know the commands to close and open are available, but I would like to have the toggle show all the time (or at least on hover) as when writing code with indents.

Proposed solution

I would like to either by default or optionally enable heading folding toggles displayed for all headings that would show expand/collapse state.

Alternatives

I haven't yet found another package that includes this feature for Markdown, though the feature is part of the core in similar circumstances.

As you can see in this screenshot, if I indent the UL, I can get the desired behavior from the heading, but obviously, that's different Markdown.

[zhuochun]

At the time when I implemented the folding command, I have not figured out how to register the toggle automatically.