Introduce build_visualization option

[planthaber]

In our Institute we had the use case to build the rock workspace without visualizations.
Also, we realized that the existing manifest.xml files in the viz subfolders are not evaluated at all, even though there is one in the official template. Was this a feature of autoproj that was possibly lost when going from autoproj v1 to v2?

This PR introduces a configuration option to select globally if the viz folders should be build and also uses the viz/manifest.xml files to be able not to install dependencies only needed for the visualization.

This PR is meant to start a discussion on how to handle this situation the best way, e.g. we were unsure if we should actually ask for the option or just have a documentation on how to disable it by setting the config option directly.

[doudou]

The viz/manifest.xml was never supported. It's a bug from the template generation script.

I kind of like the idea ... But let me sit on it for a while.

[pierrewillenbrockdfki]

There is also the option to create an Autoproj.config and then add a condition to the relevant manifest.xml dependencies, optionally also define a cmake variable that makes the rock cmake macros skip over the viz directory even if vizkit3d is present.

Edit: basically the same as Steffen did, but with the manifest.xml in the repository root controlling both dependencies, with condition="build_visualization" added to dependencies only needed for viz.