Implement copy/paste (yank in Vim lingo)

[owickstrom]

This closes #22, and as it was closely related, it also closes #17.

Make sure to describe (where applicable):

• Copy (yank) and paste with a single clipboard (register in Vim)
• Delete also puts the deleted composition part in the clipboard
• This changes some functionality by:
  • removing the leftmost/rightmost bindings
  • using a/A for add after/before, i.e. there are no longer separate bindings for prepend/append, only add before or after
• Tests are adapted, but not covered by property based test for valid focus

Checklist:

Please make sure to check the following items (that are applicable.)

• [x] Doesn't duplicate any existing PR
• [x] Automated tests added
• [x] Includes relevant user guide/documentation changes