New spanner icon, so I can add the overlay display panel shortcut back in.

README

fsl/fsleyes/controls/lightboxtoolbar.py

fsl/fsleyes/controls/orthotoolbar.py

fsl/fsleyes/controls/overlaydisplaytoolbar.py

fsl/fsleyes/icons/sources/spanner.xcf

fsl/fsleyes/icons/sources/spannerHighlight.xcf

fsl/fsleyes/icons/spanner24.png

fsl/fsleyes/icons/spanner24@2x.png

fsl/fsleyes/icons/spannerHighlight24.png

fsl/fsleyes/icons/spannerHighlight24@2x.png