XY2IJK coordinates work in progress.
X.renderer2D.xy2ijk(x,y) to convert from canvas coordinates to volume coordinates works.

TODO: Strange offset after panning.
WIP for IJK coords when the image was rotated.
Enhance the xy2ijk conversion code and enable SliceNavigators.

It is now possible to hold 'shift' while moving over a 2D renderer to control the perpendicular slices as well. This is accompanied by "slice navigators" which are shown as a cross-hair.

Unfortunately, the ijk2xy conversion (the heart of this commit) still has problems when a sliceview was rotated or panned.
Fix padding in connection with the xy2ijk functionality.
Reduce code duplication by adjusting the array indices depending on the renderer orientation.
Fix compilation and clean-up code.

Note: Rotating a 2D viewer is not working in connection with the xy2ijk function and won't be fixed because rotating soon will be deprecated.
Merge branch 'ijk'