Fix handling of thesaurus words with only a single suggestion

When there's only one suggestion for a given word the part of speech
appears in the suggested replacement, which needs to be stripped.

Closes #1776.
Fix use-after-frees when opening files from the MRU lists

Pass the paths to open by value to avoid ending up with a dangling
reference to a path stored in the MRU list which has been invalidated by
adding or removing items.

Closes #1777.