[spellcheck runner] Make each suggestion copyable as separate item

Summary:
Before all suggestions are only shown as single item with a text of
comma-separated terms. So one cannot select a single suggested term and e.g.
copy it to the clipboard

And if correct term, the term is not copyable directly as well.

Test Plan:
Activate spellcheck runner (disabled by default), enter
"spell Compilcated" or similar
and select spellchecker menu items.

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10499