0031004: Coding - eliminate warnings issued by gcc 9.1.0

Fixed -Wrestrict warning within OSD::SetSignal().

Fixed -Wdeprecated-copy warning by adding missing counterpart
in pair copy constructor / assignment operator or by removing trivial constructor.

AdvApp2Var_Node, AdvApp2Var_Patch are now declared as Handle.
AdvApp2Var_Iso is now passed by Handle.

Disabled operator= for TDF_Transaction and TDF_IDFilter.

Standard.cxx - fixed GCC version mischeck causing building failure with experimental GCC versions.

TopOpeBRepDS_EXPORT.cxx - fixed -Wmaybe-uninitialized warnings.

src/StepData/StepData_Field.hxx

@@ -57,7 +57,14 @@ public:
   
   //! Gets the copy of the values of another field
   Standard_EXPORT void CopyFrom (const StepData_Field& other);
-  
+
+  //! Assignment
+  StepData_Field& operator= (const StepData_Field& theOther)
+  {
+    CopyFrom (theOther);
+    return *this;
+  }
+
   //! Clears the field, to set it as "no value defined"
   //! Just before SetList, predeclares it as "any"
   //! A Kind can be directly set here to declare a type