Update UI after completion is applied and no linked mode is created

After the completion is applied to the underlying document, the editor
needs to be updated. Because the cursor position is independent from
the document content it needs to be updated separately, which was
forgotten in the case that no linked mode is created.

Fixes #1002151
Remove dead code in `CompletionProposal`
Merge pull request #717 from sschaef/t1002151-wrong-cursor-position-after-completion

T1002151 wrong cursor position after completion