Refactored icon management, because of stupied OSX retina display.

fsl/fslview/controls/orthotoolbar.py

fsl/fslview/controls/overlaydisplaytoolbar.py

fsl/fslview/controls/overlaylistpanel.py

fsl/fslview/icons.py

fsl/fslview/icons/axialSlice.png

fsl/fslview/icons/axialSlice@2x.png

fsl/fslview/icons/axialSlice_32.png

fsl/fslview/icons/camera.png

fsl/fslview/icons/camera@2x.png

fsl/fslview/icons/camera_32.png

fsl/fslview/icons/chainlink.png

fsl/fslview/icons/chainlink@2x.png

fsl/fslview/icons/chainlink_20.png

fsl/fslview/icons/coronalSlice.png

fsl/fslview/icons/coronalSlice@2x.png

fsl/fslview/icons/coronalSlice_32.png

fsl/fslview/icons/eye.png

fsl/fslview/icons/eye@2x.png

fsl/fslview/icons/eye_16.png

fsl/fslview/icons/eye_20.png