Configurable z-index for overlays?

[peci1]

Hi, thanks a lot for the visualization plugins you made. I really love all the overlay plugins. I'm not that familiar with 3D rendering and Ogre, so I can't figure out a pull request myself. It would really be nice to be able to configure z-index of the overlays, so that if they overlap, the order in which they are rendered is given (currently, it seems to me the order is random). There could either be a configuration value for the z-index, or it could at least be given by the order of the overlays in the Displays view (i.e. the closer to the end of the list, the higher z-index).