Debugger should have a UI gesture to evaluate properties that require all threads

## Steps to reproduce
1. Debug the following program
2. Evaluate `Program.Test` in the watch window

## Expected behavior
There should be a UI gesture to evaluate the expression

## Actual behavior
The only way to evaluate `Program.Test` is to evaluate `Program.Test, run-all-threads` from the Debug Console (NOTE: This requires a not-yet-released version of the C# extension).

## Program content
```C#
using System;

namespace clinew
{
class Program
{
static string Test
{
get
{
System.Diagnostics.Debugger.NotifyOfCrossThreadDependency();
return "Test";
}
}

static void Main(string[] args)
{
Console.WriteLine("Hello World!");
}
}
}
```