avoid initially loading value map previews from the first layer (might be expensive)
add msvc notes
[TRANSUP] string update by request

Signed-off-by: Werner Macho <werner.macho@gmail.com>
Legend refactoring: first steps towards legend customization

- map layers have associated QgsMapLayerLegend instance
- QgsMapLayerLegend is interface for generation of legend with some default implementations
- QgsLayerTreeModelLegendNode is interface for representation of legend item in layer tree model with some default implementations
Move text methods out of QgsComposerItem into QgsComposerUtil, include tests