#96 Add background image

[Philippe Poilbarbe]

Having the hability to add a background image which covers the whole label and is at the lower level layer.
The options would be:
- auto shrink/expand to cover the whole label with or without keeping proportions
- horizontal alignment : left/right/middle (when image is shrink/expanded with keeping proportions)
- vertical alignment: top/bottom/center (when image is shrink/expanded with keeping proportions)