Cover all printed text with a call to `tr`

Some of the text we can read on the app is not covered by a call to the Qt function `tr` which handles the translation if necessary.
Cover all the text so later some translations could be added.

No files or tests are named. Start by searching the C++ application sources for printed, user-visible text and existing calls to Qt's tr function. Done means all readable app text is covered by tr without changing the displayed wording, and the application still builds and runs.

Written by the indexing model from the issue text.