Adding new activities should be possible anywhere in a section

A often requested feature is the ability to insert a new course module anywhere in a section not just at the end. This is especially important for trainers who update their courses very frequently.

The API part of this can be found in MDL-75594.

I'd propose the following way to implement the UI for that feature:

simplescreenrecorder-2022-08-28_19.38.27.m4v

If you hover the space between two activities (in editing mode), the button for adding a new activity there appears (without any text) - else it's hidden. The button at the bottom stays as it is.
 

See also discussion at https://moodle.org/mod/forum/discuss.php?d=437752