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

[PaulHax]

After this PR https://github.com/InsightSoftwareConsortium/itk-viewer/pull/132

• [x] check 2D image data
• [x] Check 45 degree rotated image data
• [x] 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.
• [x] Add pick axis GUI
• [x] Window/level

Srubbing in #149