Renamed the package to ...extension.robotvisualisation

The name RoboticVisualsation can be missleading, wherefore the package
is now known under the name RobotVisualisation.

Within the package name, I also changed plugin to extension, in order to
stay consitent (see commit eb9698ef0c2ea555d306b5ff99c0d845da1a94b8
(Renamed the plugin folder to extension))

Signed-off-by: Sebastian Niemann