void MapDocument::setCurrentLayer(int index)
{
    Q_ASSERT(index >= -1 && index < mMap->layers().size());
    mCurrentLayer = index;

    /* This function always sends the following signal, even if the index
     * didn't actually change. This is because the selected index in the layer
     * table view might be out of date anyway, and would otherwise not be
     * properly updated.
     *
     * This problem happens due to the selection model not sending signals
     * about changes to its current index when it is due to insertion/removal
     * of other items. The selected item doesn't change in that case, but our
     * layer index does.
     */
    emit currentLayerChanged(mCurrentLayer);
}