[FEM] BodyHeat constraint: use unique icon

- it was not possible to distinguish the constraint from the heatFlux constraint

src/Mod/Fem/Gui/Resources/Fem.qrc

@@ -10,6 +10,7 @@
 
         <!-- gui command icons: constraints -->
         <file>icons/FEM_ConstraintBearing.svg</file>
+        <file>icons/FEM_ConstraintBodyHeatSource.svg</file>
         <file>icons/FEM_ConstraintCentrif.svg</file>
         <file>icons/FEM_ConstraintContact.svg</file>
         <file>icons/FEM_ConstraintDisplacement.svg</file>

src/Mod/Fem/femcommands/commands.py

@@ -177,7 +177,7 @@ class _ConstraintBodyHeatSource(CommandManager):
 
     def __init__(self):
         super(_ConstraintBodyHeatSource, self).__init__()
-        self.pixmap = "FEM_ConstraintHeatflux"  # the heatflux icon is used
+        self.pixmap = "FEM_ConstraintBodyHeatSource"
         self.menutext = Qt.QT_TRANSLATE_NOOP(
             "FEM_ConstraintBodyHeatSource",
             "Constraint body heat source"

src/Mod/Fem/femviewprovider/view_constraint_bodyheatsource.py

@@ -36,4 +36,4 @@ from . import view_base_femconstraint
 class VPConstraintBodyHeatSource(view_base_femconstraint.VPBaseFemConstraint):
 
     def getIcon(self):
-        return ":/icons/FEM_ConstraintHeatflux.svg"  # the heatflux icon is used
+        return ":/icons/FEM_ConstraintBodyHeatSource.svg"