[HxPopover] Consider migration to plain HTML popover

[vaclavek]

Popover is alredy part of standard:

• https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/popover
• https://html.spec.whatwg.org/multipage/popover.html

Consider migration from current bootstrap implementation to plain HTML.
Implementation is ready in all modern browsers: https://caniuse.com/mdn-api_htmlelement_popover

[hakenr]

We might consider adding an alternate plain-popover component, but the current set of components is "Bootstrap-based by design." Unless Bootstrap drops their Popover component, we will have to stick with it.