Make robots look more similar

[fmauch]

Currently, all the different versions look a bit different, some of them are darker than the others. We should make sure that they all look the same.

[gavanderhoorn]

Might be nice to see whether we could 'merge' the ROS 1 and ROS 2 versions of this into a single package.

I've been thinking about converting ROS 1 description/support packages into ROS-agnostic (ie: plain CMake) projects which just happen to install files & directories in expected places, making it possible for ROS 1 & ROS 2 utilities/scripts/etc to pick them up.

The duplication in the case of the UR descriptions specifically has always annoyed me, and there doesn't really appear to be a good reason for it (also thinking about #59).