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/App/CMakeLists.txt

@@ -82,6 +82,7 @@ SET(FemScripts_SRCS
     MechanicalMaterial.ui
     MechanicalMaterial.py
     ShowDisplacement.ui
+    FemCommands.py
     _ResultControlTaskPanel.py
     _JobControlTaskPanel.py
     _ViewProviderFemAnalysis.py