MultipleChoice: Allow editing options in the UI using a dropdown menu.

[stanislaw]

[stanislaw]

With #2273 and #2252 implemented by @thseiler, let's consider this ticket closed.

Thanks @thseiler!

[stanislaw]

The feature is already available in the StrictDoc nightly version: https://strictdoc.readthedocs.io/en/latest/latest/docs/strictdoc_01_user_guide.html#3.3-Installing-nightly-StrictDoc-as-a-Pip-package.