More update_popover() fixes

[cpsievert]

During investigation of #1011, I realized that particular issue can/should be addressed in the testing code, but also that update_popover() had a few other subtle issues:

1. update_popover(title = character(0)) currently doesn't clear the current title (because no header gets passed from server to client).
2. getOrCreateElement() was always taking fallback before selector as long as fallback is truthy. The implicit assumption here is that when the popover is visible, then fallback is falsy. I think it's better to just be explicit that if the popover is visible, we get the content from Bootstrap's .popover element. Otherwise, it comes from the children of our custom element (i.e., fallback).
3. getOrCreateElement() also wasn't correctly taking the contents of the body/header when visible. In that case, it needs to be taking the children of .popover-body/.popover-title
4. When updating just the title, it was possible to end up with 2 close buttons since we weren't accounting for the fact that "current content" might already have close button. So, now, we only add the close button when the content is being updated.

Testing notes

I've updated 316-bslib-popovers to have some better coverage of this code path in rstudio/shinycoreci#263. That PR should get merged when this one gets merged.

[gadenbuie]

Nice! This code is a lot cleaner and easier to follow than the previous version. I just have one small suggestion to tweak a comment.

[gadenbuie]

Do we query tip because changes to the popover content while the popover is open might not be reflected in this.header and this.content?

I'm thinking about it in part because this line

current elements should always be a <div style="display:contents">

makes it seems like we might have a stable element to point to, in which case this.header could hold the reference to that element directly.

Actually, after reading ahead a bit, I can see that the <div> is probably replaced on update. Maybe just a wording tweak to the comment?

Suggested change

	// These current elements should always be a <div style="display:contents">
	// with the actual content inside.
	const currentHeader = this.visible
	? (tip?.querySelector(".popover-header")?.children[0] as HTMLElement)
	: (this.header as HTMLElement);
	// We take the first child of header/content because we wrap both in a
	// <div style="display:contents"> wrapper element below.
	const currentHeader = this.visible
	? (tip?.querySelector(".popover-header")?.children[0] as HTMLElement)
	: (this.header as HTMLElement);

[cpsievert]

Do we query tip because changes to the popover content while the popover is open might not be reflected in this.header and this.content?

Yes, but also, when the tip is visible, this.header and this.content are actually empty because they move elsewhere in the DOM (specifically, from the custom element to the Bootstrap controlled .popover element).