It has been tested with Voice Over (mac) and with NVDA (windows).
When an added stamp annotation is focused, the screen reader will announce
that it's a figure containing a graphic with the added alt-text.
The doorhanger for highlighting has a basic color picker composed of 5 predefined colors
to set the default color to use.
These colors can be changed thanks to a preference for now but it's something which could
be changed in the Firefox settings in the future.
Each highlight has in its own toolbar a color picker to just change its color.
The different color pickers are so similar (modulo few differences in their styles) that
this patch introduces a new class ColorPicker which provides a color picker component
which could be reused in future editors.
All in all, a large part of this patch is dedicated to color picker itself and its style
and the rest is almost a matter of wiring the component.
