Add icon for HTML5 canvas permissions PopupNotification UI.

The image is freely licensed and obtainable from:
https://openclipart.org/image/300px/svg_to_png/21620/ben_palette.png
Add CSS whitespace hack for PopupNotification XUL <description> elements.

This code was suggested by Pearl Crescent as a fix to the XUL
<description> element of the HTML5 canvas permissions PopupNotification
not respecting whitespace in strings.

See https://trac.torproject.org/projects/tor/ticket/12684#comment:21
fixup! Make "Not now" the default for HTML5 canvas access permission dialogue.
fixup! Add icon for HTML5 canvas permissions PopupNotification UI.
Fix previous CSS whitespace hack to run before adding ServiceObserver.
fixup! fixup! Add icon for HTML5 canvas permissions PopupNotification UI.