Add Native implementation

[mgierlasinski]

PR changes:

• new project has been added MagicGradients.Native with native implementation for Android, iOS and Mac.
• new sample app: GradientsApp with native Android and iOS projects. iOS samples will be added later in separate PR.
• breaking change: CssGradientSource is now light IGradientSource implementation in Core, old xaml CssGradientSource has been renamed to CssGradient.
• builder API changes: AddCssGradient can now add multiple css gradients, it's the same as CssGradientSource.Parse.