Fixed bugs with feature count, maintenance of QgsLegendSymbolItemV2
Use QgsLayerTreeModel as a source of data for legend rendering instead of QgsLegendModel

This is work in progress, customization of legend is not working yet
Moved QgsLayerTreeModelLegendNode (+derived classes) to a new file
Support for custom labels in symbol-v2 legend node
New flag to disable checkboxes for legend nodes in layer tree model
Layer tree model support replacement of layer tree + layer node handles removal of layer cleanly
Work in progress: Make composer legend widget work with QgsLayerTreeModel