Append newline at QtSignalChannel level rather than MessageDisplay

The Poco log stream emits the log messages when it receives a newline but
doesn't send along newlines. However, printing from Python scripts does
produce newline characters so in order not to double-count them it makes
most sense to simply emit a newline in the SignalChannel and let
everything listening to it simply insert the exact text received.

Refs #7152
Merge remote-tracking branch 'origin/bugfix/7152_double_spaced_resultslog'