Implemented project multitargeting and macOS

[fedemkr]

Done:

• Changed main project to multitargeting: xamarin.ios10;xamarin.mac20
• Adapted iOS implementation
• Implemented macOS fluent layouts
• Added macOS QuickLayout sample
• Solves #54

To Do:

• Update build script
• Implement macOS RightToLeftView -> RTL behavior
• If this is OK, we can add the target for tvOS and somehow merge #49

[fedemkr]

Hi there, I can't enter the bitrise details. Could you tell me what is wrong? so that I can fix this.