Update composer legend entry when layer symbology changes. 

This commit ensures that the composer legend item symbols are correctly update when the symbology of the respective layers change, if automatic updates to the composer legend are enabled.

There is one slightly ugly aspect, namely that the actual layer is retrieved via `QObject::sender()`. Another option would have been to add a `QgsMapLayer::rendererChanged(QString)` signal which is emitted like  `QgsMapLayer::rendererChanged()`, but also passes the layer id. However, it adds more code.

Funded by Sourcepole QGIS Enterprise.