Add Line to elements header

https://gitlab.com/ArmarX/RobotAPI/-/commit/aa89a38f9e2dcc247353642bf3735862852ccbf9 removed the option to use viz::Polygon as lines between two points. While it seems sensible to force users to use the better suited viz::Line instead, the latter is not yet in the header that includes the standard drawing elements. Hence auto completion does not show the existence of viz::Line, which is addressed by this merge request.

A recent change to the elements header is https://gitlab.com/ArmarX/RobotAPI/-/commit/b2aff28ff4f3e29f4d5aa8b228839fdceb979d9b, which already added viz::Path. As far as I can see, viz::Line is the only "simple" element type that is still missing.