Allow overridable and overriding events

Given the following base class definition (in C#, since VB doesn't allow declaring virtual/overridable events, AFAICT):

```
public class Calculator
{
public virtual event EventHandler TurnedOn;
}
```

The following is perfectly valid in C#:

```
public class CalculatorProxy : Calculator
{
public override event EventHandler TurnedOn
{
add => // do something
remove => // do something
}
}
```

Yet the same cannot be achieved with VB, neither with an `Overrides Event...` nor a `Overrides Custom Event` or the like.

Even if declaring the `Overridable` event isn't allowed, for parity at least the `Overrides` custom event could look something like:

```
Public Class CalculatorProxy
Inherits Calculator

Public Overrides Custom Event TurnedOn As EventHandler
AddHandler(value As EventHandler)
` do something
End AddHandler

RemoveHandler(value As EventHandler)
` do something
End RemoveHandler

RaiseEvent(sender As Object, args As EventArgs)
` do something
End RaiseEvent
End Event
End Class
```