Stunner - Support for image strips

The goal for this task is moving to the use of image strips (also known for CSS as "sprite") instead of using separate data-uri's for each image.

This way performance and memory consumption is being improved as:

• On the canvas side there is only need to perform a single DOM image load operation
• On the HTML/DOM side the image tags can be replaced by just div's and just use CSS declarations in order to clip the image

Also as a result of this task, the grey boxes that appear for a short time in some browsers (FF), while the toolbox is loading the icons, will not appear anymore, as the image (strip) will be already loaded. See the attached screenshot that shows the actual behavior, before this fix, where grey boxes appear while icons are loading