refs #1576: fixed icon for Modelica.Thermal.HeatTransfer.Fahrenheit.FixedTemperature, Modelica.Thermal.HeatTransfer.Fahrenheit.PrescribedTemperature and Modelica.Thermal.HeatTransfer.Fahrenheit.TemperatureSensor

git-svn-id: https://svn.modelica.org/projects/Modelica/branches/maintenance/3.2.1@7867 7ce873d0-865f-4ce7-a662-4bb36ea78beb