Allow removal of urdf elements through propertywidget context menu

[gavanderhoorn]

The currently implemented context menus allow a user to add elements like visual and collision, but they can't be deleted yet.

This should be added. It should be possible to delete all elements.

[gavanderhoorn]

TODO: should the UI allow the user to end up with illegal URDFs (ie: through deleting elements that are actually required in a minimal urdf)?

[Kukanani]

I already see both Add and Remove on the context menus when I right-click on a link in the property browser. Is this a different issue?

[gavanderhoorn]

Yes: the UI has both a tree and a list view. The 'property widget' is the list view. That only allows/ed adding properties, not removing them.

Additionally: this issue talks about properties, which links and joints are not (they have properties, but that is something different).