Element picker tweaks

- Indentation whitespace fixes.
- Use built-in getBoundingClientRect() function instead of self-made.
- Use built-in DOM API for manipulating the class attributes instead of
  altering the className property.
- Add pointer-events: none to the svgRoot when using
  document.elementFromPoint(), but if the browser (older Safari for
  example) doesn't take the pointer-events into account, then fall back to
  display: none.
- Initiate every part of the picker at the same time; when the message is
  received from the background. This way the selected element will have
  the red overlay immediately, instead of showing first the black overlay,
  then a few milliseconds later the red.
"Purge all caches" button seemingly didn't work