Task: Default to text editor font for monospaced text

This used to be hardcoded to "Monospaced" which does not work on Mac
(not even with the provided hint to choose a monospaced font).

Task-number: QTCREATORBUG-11345
Change-Id: I5d72ba9e971933fef8ef94c20333035188d33a57
Reviewed-by: Eike Ziller <eike.ziller@digia.com>