feature: Added Uno support

[weitzhandler]

This PR adds initial support for the Uno Platform.

[weitzhandler]

Looks like generic type arguments are not supported.

Opened suggestion.

Anyway the solution I'm thinking of, is having on each view code-behind a dummy class that inherits from ReactiveUserControl<TViewModel> and is used as the XAML subclass, for example:

<v:ReactiveMainPage
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:v="using:MyProject.Views"
  x:Class="MyProject.Views.MainPage"  
/>

namespace MyProject.Views
{
  public class ReactiveMainPage  : ReactiveUserControl<MainPageViewModel>
  { 
  }

  public sealed partial class MainPage : ReactiveMainPage
  {
  }
}

I'd be happy to hear about any other ideas of course.

[glennawatson]

ReactiveUI/src/ReactiveUI/Platforms/windows-common/ReactiveUserControl.cs

Line 45 in e6e8b3c

/// Note that UWP projects do not support the <c>TypeArguments</c> attribute. The XAML designer window in WPF projects also does not

that's the current proposal we have for users in our docs.

[weitzhandler]

@jeromelaban can u take a look around here?

[jeromelaban]

Looks good to me!

[weitzhandler]

@glennawatson Perhaps revert this commit, I opened #2078 for that.

[glennawatson]

Leave it in, I'm going to check some stuff the blazer guys did which would affect that portion of the code.

[weitzhandler]

Thank you guys for the hard work, you're amazing!