View 2D MVP: Off axis oriented images, Slice axis, Slice slider, Window/Level control

[PaulHax]

After this PR #132

• check 2D image data
• Check 45 degree rotated image data
• Check up direction for each axis.
• Show slice index in GUI slider, rather than 0 to 1 float. If slicing mode is Z, the setSlice is in world coords not index???
• Check that GUI slider allows selection of min and max slice. Does not round out top or bottom slice.
• Add pick axis GUI
• Window/level