FEM: some renameing for multiple solver:

- module names vs multiple solver
- stick with python command class names in Fem
- rename commands for new objects from _CommandFemSomeObjName to _CommandSomeObjName
- rename commands for do something to _CommandDoSomething
- change tool names to have a consistent naming in all FEM for menue tools and tool tipps

src/Mod/Fem/_TaskPanelFemShellThickness.py

@@ -87,8 +87,8 @@ class _TaskPanelFemShellThickness:
         FreeCADGui.Selection.clearSelection()
         # start SelectionObserver and parse the function to add the References to the widget
         print_message = "Select Faces by single click on them to add them to the list"
-        import SelectionObserverFem
-        self.sel_server = SelectionObserverFem.SelectionObserverFem(self.selectionParser, print_message)
+        import FemSelectionObserver
+        self.sel_server = FemSelectionObserver.FemSelectionObserver(self.selectionParser, print_message)
 
     def selectionParser(self, selection):
         # print('selection: ', selection[0].Shape.ShapeType, '  ', selection[0].Name, '  ', selection[1])