Symbol display control

Fix for issue #57

Added a new SymbolDisplay control which displays a symbol by taking care of the current view DPI.

The control is initialized from a Symbol and creates internally the swatch by using Symbol.CreateSwatchAsync().
The symbol is displayed in order to avoid any clipping:
- if the available size is greater than the symbol size, the symbol is displayed at scale (example of use case: legend control that uses a SymbolDisplay without specifying the Height/Width)
- if the available size is smaller than the symbol size, the symbol is streched to fit in the available size (example of use case: TemplatePicker control that uses a SymbolDisplay with a fixed Height/Width)

Other optional property:
  - GeometryType --> by default the geometry type is deduced from the symbol type