Fixed to match Credential API refactoring.
SymbolDisplay Initial implementation
WinStore/WinPhone SymbolDisplay: Subscribe to DpiChanged events
Added DefaultHeight and DefaultWidth properties.
Declare temporarily Symbol as object (workaround to Symbol not bindable)
WinPhone: Don't subscribe to DpiChanged event (useless)
WinStore: Use Loaded/Unloaded events to subscribe to DpiChanged event
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
TemplatePicker based on SymbolDisplay:

  - TemplateItem (private class but used for Binding):
      . Removed Swatch Property
      . Added GeometryType and Symbol properties
  - Default template picker styles based on  SymbolDisplay
  - Use Loaded/Unloaded events to subscribe/unsubscribe to layers events
Merge remote-tracking branch 'origin/master' into dom/issue57_SymbolDisplay
Moved SymbolDisplay to Primitives namespace.
Removed use of ThottleTimer.
Removes previous temporary workaround to API issue.
Get rid of the ArrangeOverride by setting MaxHeight/MaxWidth instead of Height/Width
Merge branch 'dom/issue57_SymbolDisplay' into dom/Legend_UseSymbolDisplay
Updated due to SymbolDisplay namespace change.
Merge branch 'dom/issue57_SymbolDisplay' into dom/TemplatePicker_UseSymbolDisplay
Updated due to SymbolDisplay namespace change.
Added WinPhone and Desktop OAuthAuthorize component.
Changed namespace from Esri.ArcGISRuntime.Toolkit.Primitives to Esri.ArcGISRuntime.Toolkit.Controls.Primitives for consistency with LegendItemViewModel namespace.
Merge branch 'v.next' into dom/issue64_OAuthAuthorizeComponents
Merge branch 'v.next' into dom/api_credential_refactoring
Merge branch 'v.next' into dom/issue57_SymbolDisplay