Added all div marker images. Note that the images were edited in external program...

Added all div marker images. Note that the images were edited in external program in order to exploit pngs transparent background capabilities: this lets the pngs to be independent from future potential div background changes (actually, this kind of editing should be done for all pngs in the page). Implemented the edit button and consequent hiding/showing of the marker description textarea. Added the label showing the marker description (for which a caomputation of the available size is required: this part is still to be terminated)