Lots of code which tests for Image overlays now needs to test for

fsl/data/strings.py

fsl/data/tensorimage.py

fsl/fsleyes/controls/canvassettingspanel.py

fsl/fsleyes/controls/clusterpanel.py

fsl/fsleyes/controls/lightboxtoolbar.py

fsl/fsleyes/controls/locationpanel.py

fsl/fsleyes/controls/orthotoolbar.py

fsl/fsleyes/controls/overlayinfopanel.py

fsl/fsleyes/displaycontext/display.py

fsl/fsleyes/displaycontext/displaycontext.py

fsl/fsleyes/displaycontext/modelopts.py

fsl/fsleyes/gl/slicecanvas.py

fsl/fsleyes/views/canvaspanel.py

fsl/fsleyes/views/orthopanel.py

fsl/tools/render.py