Legend based on SymbolDisplay:

   - LegendItemViewModel.SymbolSwatch property removed
   - LegendItemViewModel.GeometryType property made public
   - LayerItemViewModel/Legend optimized to avoid useless LayerItemsSource changes (and so useless swatch creations)
   - LegendItemTemplate based on SymbolDisplay
Merge branch 'dom/issue57_SymbolDisplay' into dom/Legend_UseSymbolDisplay
Updated due to SymbolDisplay namespace change.
Merge branch 'dom/issue57_SymbolDisplay' into dom/Legend_UseSymbolDisplay
Remove setting of DefaultWidth and DefaultHeight.
Merge branch 'dom/issue57_SymbolDisplay' into dom/Legend_UseSymbolDisplay
Merge pull request #79 from Esri/dom/Legend_UseSymbolDisplay

Legend based on SymbolDisplay