Means URLs like :/icons/foo.jpg should become icons:foo.jpg
This is part of the prep work for a PyQt6 update. PyQt6 has dropped
pyrcc, so we can longer generate the icons_qrc.py file we did previously.
Qt Designer expects us to use the resource system, so we continue to
generate the icons.qrc file to make editing the UI files easier. But at
runtime, we no longer use that file.
