Feature: Background Images Origin Tool

[Onetchou]

The tool allows the user to select an origin point on the image.

This point:

• (MVP) should be the point whose coordinates are set with the x and y spinboxes in the properties dialog
• (MVP) should be the rotation point of the image when rotated with the properties dialog (so that rotating the image doesn't change its position)
• (ADVANCED) can be snapped to a point in the pattern / to nothing : add a combobox in the image dialog.

The user selects this tool, then clicks on the image, and it displays the origin on the image with a symbol to be determined.

[Onetchou]

Add a tool in the image toolbox: