FEM: material editor, add command to open the editor ot icon toolbar, use copy of Arch material group icon

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

@@ -1,5 +1,7 @@
 <RCC>
     <qresource>
+        <file>icons/Arch_Material_Group.svg</file>
+
         <file>icons/FemWorkbench.svg</file>
         <file>icons/preferences-fem.svg</file>
 

src/Mod/Fem/Gui/Workbench.cpp

@@ -68,6 +68,8 @@ Gui::ToolBarItem* Workbench::setupToolBars() const
            << "FEM_MaterialSolid"
            << "FEM_MaterialFluid"
            << "FEM_MaterialMechanicalNonlinear"
+           << "FEM_MaterialEditor"
+           << "Separator"
            << "FEM_ElementGeometry1D"
            << "FEM_ElementRotation1D"
            << "FEM_ElementGeometry2D"

src/Mod/Fem/femcommands/commands.py

@@ -307,7 +307,7 @@ class _CommandFemMaterialEditor(CommandManager):
     "The FEM_MaterialEditor command definition"
     def __init__(self):
         super(_CommandFemMaterialEditor, self).__init__()
-        self.resources = {'Pixmap': 'fem-femmesh-clear-mesh',
+        self.resources = {'Pixmap': 'Arch_Material_Group',
                           'MenuText': QtCore.QT_TRANSLATE_NOOP("Material Editor", "opens the FreeCAD material editor"),
                           # 'Accel': "Z, Z",
                           'ToolTip': QtCore.QT_TRANSLATE_NOOP("Material Editor", "opens the FreeCAD material editor")}