List of substantive changes.

 - RViz plugins now use ROS pluginlib.
   - RViz plugins cannot be loaded and unloaded individually.  RViz
     attempts to load all available plugins every time.
     - Plugin Manager menu entry and dialog are gone.
     - If need be, I can make a scheme for preventing certain plugins
       from loading, but there is no function in pluginlib to unload a
       library.

 - "Manage Displays" button is replaced by "Rename Display" button,
   which just renames the currently-selected display.

 - To reorder Displays, just drag them with the left button.

 - Multiple Displays can be selected and dragged or deleted together.
