public final void clearOverlays() {
        LOGGER.trace("clearOverlays");
        nonBlockingOverlaysShown.clear();
        blockingOverlaysShown.clear();
        overlays.clear();
    }
Removes all previously loaded overlays from the scene graph including all references to them,
in order to free up memory.
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/Workbench.java