Debugger: Improve the helper build mechanism on the settings page.

- Give the controls a tooltip listing file details (date)
- Make building a QtConcurrent task
- Make log window scroll to bottom and pop up on error
- Make the build code pass on error messages about copying the
  source files to the log file
- Clean up the building code string-wise, use QLatin1String and
  translate messages, cache the icons. Use a QSharedPointer for
  the QtVersions in the settings page since they are passed to
  the BuildTask, which may outlive the dialog.