rustdoc: improve popover focus handling JS

This commit fixes a few inconsistencies and erratic behavior from the
notable traits, settings, and sidebar popups:

* It makes it so that pressing Escape closes the mobile sidebar.
  This is a bit difficult to do on iPhone, but on other setups like
  desktop tiling window managers, it's easy and makes sense.
* It makes sure that pressing escape while a notable trait popover is
  open focuses the popover's toggle button, instead of leaving nothing
  focused, since that makes more sense with keyboard navigation. Clicking
  the settings, help, or sidebar buttons, however, will not focus the
  notable trait popover toggle button.
* It ensures that notable trait and settings popovers are exclusive
  with the mobile sidebar. Nothing should ever overlap a popover, and
  there should never be more than one popover open at once.

0 files changed, 0 insertions, 0 deletions