[Debug Variables view] Allow to add columns for other running launches

Let's make sure issue is not already fixed in latest builds first.

- [x] I verified [latest Integration Build of Eclipse SDK](https://download.eclipse.org/eclipse/downloads/) doesn't provide the feature.

## Suggestion

The goal is to facilitate comparative debugging: I'm the same application from the same project with different options, and I want to more easily compare their execution.

From a fresh installation and clean workspace:

* Create a .java project with a file
```java
public class Foo {
public static void main(String[] args) {
long n = System.currentTimeMillis();
n++;
}
}
```
* Put a breakpoint on the `n++` line
* Right-click > Debug As > Java application
* Again, Right-click > Debug As > Java application

I reach state: both processes are stopped on breakpoint, at the same line, I see them in the "Debug" view

And would like to be able to: Compare instantly the value of `n` in both executions.
This could be achieved by allowing the "Variables" view to add a new "value" column for the specific launch. An action such as "Show value for " or "Show value for " would be perfect.

## Community

- [x] I understand suggesting an enhancement doesn't mandate anyone to implement it. Other contributors may consider this suggestion, or not, at their own convenience. The most efficient way to get it fixed is that I implement it myself and contribute it back as a good quality patch to the project.