Creating a "metapackage" containing a strands_ui.launch file

* The `strands_ui` package is not a metapackage in the literal sense of the word but has the same functionality + installs a launch file for mary and the webserver. Since metapackages are not allowed to install things it's a regualar package.
* The `strands_ui.launch` file launches mary and the webserver together
* The `strands_ui` package does not include the marathon gui because I didn't know if that will be released in the first place.
* The `nav_help_*` launch files now have a parameter that allows to start the webserver and mary together with the help nodes. This has just been added for backwards compatibility but the default value for the parameters is `false` to bring to peoples attention that the structure has changed.

All this entails, that after this is PR merged the help system won't work any more without making adjustments to the current nav launch files.