QgsMapLayerRegistry add/remove signals reworked
* "theEmitSignal" parameter renamed to addToLegend
* layer(s)WillBeRemoved has no more "theEmitSingal" parameter
* layer(s)WillBeRemoved and layer(s)(Was)Added emitted in every case
* legendLayersAdded( QList<QgsMapLayer*> theMapLayers ) signal added
* removedAll renamed to removeAll (is emitted BEFORE layers are removed)