[Proposal] Built-in MVVM pattern

I've been thinking for a year, that why VB isn't paid attention by MS (but F# do!). Now I think that it is because VB is very much like C#, and it is more easy for a C/C++/Java programmer to learn C# than VB. After they have learned C#, they found they needn't learn VB at all - there's nearly nothing that VB can do but C# cannot! Therefore, I suggest we should think about more amazing and crazy ideas to save VB from being marginalized and discarded.

Usually it is a bit complicated to write a observable class in VB. To simplify it, I suggest a new API: `ObservablePropertyAttribute`, and we can simply write
``` vb.net
Class MyObservableClass

Public Property Title As String
End Class
```
And it will be expanded by the compiler to
``` vb.net
Class MyObservableClass
Inherits ObservableObject
Private _Title As String
Public Property Title As String
Get
Return _Title
End Get
Set(value As String)
SetProperty(_Title, value)
End Set
End Property
End Class
```
where `ObservableObject` is also a new pre-defined class just like [this](https://github.com/jamesmontemagno/mvvm-helpers/blob/master/MvvmHelpers/ObservableObject.cs).

A class with `ObservableProperty` property can only inherits `ObservableObject` or its derived class. `ObservableObject` should be a public class in a independent package.

If you want a callback or validation, you can declare the functions and pass them to the attribute like
``` vb.net
Class MyObservableClass

Public Property Title As String
' Sub TitleChanged and Function TitleValidation
End Class
```
And it'll be expanded to
``` vb.net
Class MyObservableClass
Inherits ObservableObject
Private _Title As String
Public Property Title As String
Get
Return _Title
End Get
Set(value As String)
SetProperty(_Title, value, onChanged:=AddressOf TitleChanged, validateValue:=AddressOf TitleValidation)
End Set
End Property
End Class
```

Sometimes we don't write an observable object but a dependency object. I suggest a new API: `DependencyPropertyAttribute`
``` vb.net
Class MyDependencyClass

Public Property Title As String = "Hello world!"
End Class
```
And it'll be expanded to
``` vb.net
Class MyDependencyClass
Inherits DependencyObject
Public Shared Readonly TitleProperty As DependencyProperty = DependencyProperty.Register(NameOf(Title), GetType(String), GetType(MyDependencyClass), New PropertyMetadata("Hello world!"))
Public Property Title As String
Get
Return GetValue(TitleProperty)
End Get
Set(value As String)
SetValue(TitleProperty, value)
End Set
End Property
End Class
```
The types `DependencyObject`, `DependencyProperty`, `PropertyMetadata` may be different when targeting UWP or WPF, and it could be specified by command line args to vbc.