Add support for existing SimulationContexts

[LinqLover]

At the moment, the TraceDebugger will eliminate any other custom context customizations in the code to be traced. For instance, the following does not run the code sandboxed when you trace it and step over the assignment:

| x |
x := 0.
Sandbox2 debug: [x := 1].

The cause for this behavior lies in TDBTrace>>#enableSimulatorDuring: where the customizations of all contexts are via simulator customize: context. We should preserve other custom context simulators and combine all simulators instead via the COR mechanism.