Resolve "(graphics): add slices lines on SNegato2D"
Description
This adds two lines in the 2D negato views indicating where are located the two other slices.
Extra changes
- Fixes the refresh of the "yellow" cross when picking voxels
- Enhanced the layout of the views: background of the sliders and thickness of the border views
- Removed the slice slider in the 3D view (confusing)
Closes #656 (closed)
How to test it?
- Run SightViewer
- Load an image
- Play with image slices sliders in all planes and observe there are two extra lines in each plane