v2: represent element as soon as drag enters canvas
Story: As a user I want to know how the element I will add is like so I can drop it at the right place
Current State: We drag a HTML rectangle of consistent size for each element, how it actually is like is only seen later.
related: #63 ("v2 code quality: Event initialization is messy, needs lots of files to be touched")
Note: …I try with canvas.onDragEnter but so far, I did not get the event handler called when I drag the "usual" jquery element from the outside (aka sidebar). Maybe I need to use the normal draggable of jQueryUI on the canvas and then create an element.
…another way would be just using plain "mouseenter" and checking if the current drag is of the right kind aka dragging an QuickMocukup UI-Element.