Resolve "enh(filter): add a slider to navigate along the reslice plane"

Description

This adds a slider in each view of ex_image_reslice allowing to slide in the direction of the reslice. In this example, the reslice transform is directly manipulated, so it is not really useful. But in real applications, the reslice transform is likely to come from a non-editable data, for instance a sensor location. In such cases, the slider allows us to explore around the reslice direction, like if we had a real axis.

How to test it?

• Launch ex_image_reslice
• Load an image, preferably with an origin and orientation
• Play with the reslice position in the right panel
• Play with the slider in each view, it allows you to navigate in the direction of the reslice, only if the current reslice position is in the boundary of the image. When sliding, it stays inside the boundaries of the image.
• You can also test the "center" option that allows to center the reslice instead of doing it from the bottom left corner

Decreased code coverage

The plane slicer was already untested... I added a significant portion of code thus the coverage decreases. We could open a ticket to implement that later. Finally I added a new test.

Related issue(s)

Closes #1347 (closed)