FEM: Use default True value for material dirs

Defaulting to False was causing problems during migration. The default
checkbox of linked FEM preferences is True, but for the first time users
there was nothing stored in UseBuiltInMaterials, UseMaterialsFromConfigDir
and UseMaterialsFromCustomDir, so the materials were not showing up in
the material ComboBox.

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

src/Mod/Fem/MechanicalMaterial.py

@@ -250,17 +250,17 @@ def import_materials(self):
         self.pathList = []
         self.form.cb_materials.clear()
         self.fem_preferences = FreeCAD.ParamGet("User parameter:BaseApp/Preferences/Mod/Fem")
-        use_built_in_materials = self.fem_preferences.GetBool("UseBuiltInMaterials")
+        use_built_in_materials = self.fem_preferences.GetBool("UseBuiltInMaterials", True)
         if use_built_in_materials:
             system_mat_dir = FreeCAD.getResourceDir() + "/Mod/Material/StandardMaterial"
             self.add_mat_dir(system_mat_dir, ":/icons/freecad.svg")
 
-        use_mat_from_config_dir = self.fem_preferences.GetBool("UseMaterialsFromConfigDir")
+        use_mat_from_config_dir = self.fem_preferences.GetBool("UseMaterialsFromConfigDir", True)
         if use_mat_from_config_dir:
             user_mat_dirname = FreeCAD.getUserAppDataDir() + "Materials"
             self.add_mat_dir(user_mat_dirname, ":/icons/preferences-general.svg")
 
-        use_mat_from_custom_dir = self.fem_preferences.GetBool("UseMaterialsFromCustomDir")
+        use_mat_from_custom_dir = self.fem_preferences.GetBool("UseMaterialsFromCustomDir", True)
         if use_mat_from_custom_dir:
             custom_mat_dir = self.fem_preferences.GetString("CustomMaterialsDir","")
             self.add_mat_dir(custom_mat_dir, ":/icons/user.svg")