Do not allow trailing/leading whitespace in mandatory fields

Reference, Value, Footprint and Datasheet all reference elements that
should not use trailing/leading whitespace or similar hidden charaters.
Other fields may utilize leading/trailing whitespace for obscure
purposes but won't break functionality as they are display only

eeschema/sch_field.cpp

@@ -1154,7 +1154,11 @@ void SCH_FIELD::SetText( const wxString& aText )
     if( m_isNamedVariable )
         return;
 
-    EDA_TEXT::SetText( aText );
+    // Mandatory fields should not have leading or trailing whitespace.
+    if( IsMandatory() )
+        EDA_TEXT::SetText( aText.Strip( wxString::both ) );
+    else
+        EDA_TEXT::SetText( aText );
 }