Put markers in the browser chrome

Put markers in the browser chrome

Instead of inside the current page. Specifically, the markers are now inserted
at the same place as `alert()` dialogs are. This has a number of advantages:

- We don't need to fight the `z-index`es of the page. The markers will now
  always be on top.
- We don't need to fight any other styling of the page. No reset rules needed.
  The markers will look the same regardless of what page you view.
- The above point means that we can drop `!important` from all declarations.
  This makes it easier to change the styling of the markers using userChrome.css
  or Stylish.
- Just like you cannot interact with the page (such as scrolling) while an
  `alert()` dialog is visible, you cannot interact with the page when hints are
  visible. For example you can no longer scroll, making the hints out of place.