Add a 'code behind' view/tab

[gavanderhoorn]

Extend the UI with a view/tab that provides direct access to the urdf/xacro document that is currently being edited.

Full functionality of all aspects of the application should be maintained (ie: real-time model updates, property browser / tree widget triggering), but some delay is acceptable (to avoid too frequent parsing of the urdf fi).

This would allow users preferring not to use the UI for some things to stay in the application and not having to resort to use a separate text editor.

[git-jkb]

From our perspective, this would be one of the coolest features to really enhance this utility. I would say it would be quite exciting, especially if it could switch between a URDF or xacro representation.

-Josh K.