The snippets page should not operate on the font settings
changed signal when the ui does not exist.

Task-number: QTCREATORBUG-5173
Change-Id: Ic1363cb41340b5ea04803eb27de3dd0af6ecda0c
Reviewed-on: http://codereview.qt.nokia.com/511


Reviewed-by: Eike Ziller <eike.ziller@nokia.com>