FEM: Add FemCommand class and use it in _CommandFrequencyAnalysis

FemCommand class will gather all common functions/propertied od FEM gui
commands. That should allow to reduce some common code.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>

src/Mod/Fem/Gui/AppFemGui.cpp

@@ -102,6 +102,8 @@ void FemGuiExport initFemGui()
     FemGui::ViewProviderResult                 ::init();
     FemGui::ViewProviderResultPython           ::init();
 
+    Base::Interpreter().loadModule("FemCommands");
+
     Base::Interpreter().loadModule("_CommandMechanicalShowResult");
     Base::Interpreter().loadModule("_CommandFrequencyAnalysis");
     Base::Interpreter().loadModule("_CommandQuickAnalysis");