Added go to slide linking for elements.
Made it easier to add elements that are not separate H5P libraries.
Added internal go-to-slide element. Making it easy to jump between slides.
Cleaned up order in semantics, making required element options come first.
Avoid error when going to non-existing slides.
Merge branch 'go-to-slide' of github.com:h5p/h5p-course-presentation into go-to-slide
Resolve conflicts
Merge branch 'go-to-slide'