Add support for Linux, WPF, Android, macOS, and iOS

[Arlodotexe]

This PR:

• Makes progress towards Full support for Uno Platform #28
• Adds Labs.SampleRefs.props that can be used to include all sample project references in a project head.
• Adds xPlatforms solution folder to hold all platform heads without cluttering the solution root
• Adds default cross platform heads for Droid, iOS, macOS, Skia.Gtk (Linux), Skia.WPF, and Skia.Wpf.Host.
  • macOS and iOS are untested due to a lack of hardware (thanks Apple)
  • All projects retain the default project name and Uno branding. An issue will be filed in this repo shortly after this PR is merged to update branding.

[michael-hawker]

I get these errors trying to compile:

Am I missing an environment thing now?

[michael-hawker]

Ran Maui-check and Uno-check and then got my machine working, filing an issue #46 as we discussed to simplify this for the base case.

For now, LGTM. Tested UWP, WinAppSDK, and Android. (CanvasLayout worked perfectly there, imagine that!)