Add duplicate and rename buttons to plugin panel

[rjb0026]

It would be nice to have all the same plugin control buttons that RVIZ has at the bottom of the plugin control panel. Currently mapviz has:

• Add
• Remove

And RVIZ has:

• Add
• Duplicate
• Remove
• Rename

The "Duplicate" feature is proposed with #736 and renaming is already possible but must be done by right clicking the plugin and selecting "Edit name". Exposing these features as buttons will make learning the UI of mapviz much easier, especially for those already familiar with RVIZ.