Feature: Background Images Origin Tool

[Onetchou]

This tool should allow the user to change the default top-left origin point of the image to a custom point.
The user clicks on the tool button, then clicks on the image where he wants the origin point to be.

This point:

• should be the point whose coordinates are set with the x and y spinboxes in the properties dialog
• should be the rotation point of the image when rotated with the properties dialog (so that rotating the image doesn't change its position)

An "origin point" symbol should be displayed on the image to show its position.