LibWeb: Hide unrelated popovers when showing popovers

[Gingeh]

Also hides decendant popovers when hiding.
Also hides unrelated popovers when showing dialogs.

This PR is affected by the following spec bugs, each instance of ad-hoc behaviour is noted and referenced:

• Should the popoverList in "hide popover stack until" and "close entire popover list" be dynamically updated? whatwg/html#11007
• Assertion in "hide all popovers until" will fail when changing a showing "hint" popover to "auto" whatwg/html#10996
• The topmost popover ancestor algorithm is used for manual popovers whatwg/html#10988
• Hint popovers can't be the "topmost open popover ancestor" of dialog elements whatwg/html#11008
• Ignore DOM state when hiding popovers if needed whatwg/html#9457