Allow zoom and move when editing layer

[schwobr]

It seems that in some cases it is quite unpractical to annotate while having the position and resolution of the image fixed. To allow that we would need event handlers for mouse wheel and for concurrent mouse left click and moving. It would need to scale and move both the background image and the svg polygon. I assume this should go in Layer.js somewhere but I am not quite sure how to:

• Modify the background image from this file (I assume it is not currently possible)
• Handle concurrent events

Do you have any idea on this @ArnaudAbreu ?

EDIT: Moving should probably use onDrag event