Improve the Gulpfile

[timvandermeij]

Recently we upgraded to Gulp 4. However, the following things still need to be addressed in some way:

• If a task series is executed with an anonymous function inside it, we'll see Starting '<anonymous>' in the output, which isn't that great. We need to either suppress it or use named functions.
• If a task is private, it should be hidden in some way. For example, gh-pages-prepare is only used in the public web target and should not be used on its own, however currently it shows in the default gulp output since it's a regular task. Perhaps we should mark them with an underscore?