[FEATURE] Default values on update

Add an option to add default values on update of attributes or geometry.
This can be seen as a "derived value" that will be saved to the database
(like area of a polygon) or as a means to track last_modified dates etc.

See #5218

python/core/qgsdefaultvalue.sip

@@ -41,6 +41,7 @@ class QgsDefaultValue
     QgsDefaultValue( const QString &expression = QString(), bool applyOnUpdate = false );
 %Docstring
  Create a new default value with the given ``expression`` and ``applyOnUpdate`` flag.
+.. seealso:: QgsVectorLayer.setDefaultValueDefinition
 %End
     bool operator==( const QgsDefaultValue &other ) const;
 

src/core/qgsdefaultvalue.h

@@ -55,6 +55,7 @@ class CORE_EXPORT QgsDefaultValue
 
     /**
      * Create a new default value with the given \a expression and \a applyOnUpdate flag.
+     * \see QgsVectorLayer::setDefaultValueDefinition
      */
     QgsDefaultValue( const QString &expression = QString(), bool applyOnUpdate = false );
     bool operator==( const QgsDefaultValue &other ) const;