Higher level helper functions

With #only and #uncover you can come a long way but there are some reoccurring situations for which helper functions are provided. We call them "higher level" because they use #only and #uncover under the hood and operate on larger pieces of content.

For the common case of succesively revealing content, there are #pause and #one-by-one and its friends. For substituting content, we have #alternatives in different variants. The following sections will describe these functions in detail.