Reference layer/image

[WallK]

As a pattern designer
I want to put an image begind the pattern
So I can put stitches by referencing the image

Acceptance criteria

User should be able to select an image (supported formats) on their hard drive
This image should be fitted (preserve aspect ratio) to the canvas
Reference image is displayed behind the pattern
Available settings:
    Reference size (WxH in stitches?)
    Transparency (0-100%)
    Position offset (from top left, in stitches?)