Restructure sources & add readme files in every folder

     - The files tree in `/sources/` has been altered;

     - two new icons have been included: `write` and `inbox`;

     - included `readme.md` files in every folder to better explain the
       project;

     - all icons have been merged together into a single SVG file at
       `source/icons/set.svg`;

     - the style sheets `toolset-icons.scss` and `toolset-contents.scss`
       have been merged together into a new style sheet called
       `toolsets.scss`;

     - styling contained in `icon-elements.scss` has been altered and
       moved to `source/icons/styling.scss`, the file will be compiled and
       inserted into the contents of `source/icons/set.svg` on the build
       script, the result will be a SVG file named `icons.svg` that is
       going to be included into the .xpi file;

     - previosly, the logic in `bootstrap.js` included the icons' SVG
       content into the targeted windows' DOM and the windows' style sheet
       had rules targeted to that content, the image data was then
       retrieved via the `-moz-element(...)` function; because the
       icons' styling is now going to be directly inserted into the
       SVG content by the build script, the SVG content is not going to be
       included into the DOM anymore and the image data will be simply
       retrieved via the `url(...)` function.