Debugger: Cleanups.

Get rid of ExpressionEvaluationWindow.

- When asking to evaluate an expression via the Tools menu, we now
  bring up a prompt window the same as the one used to add a watch
  expression. This in turn works exactly the same as the latter,
  except an additional flag is sent indicating that the expression
  in question should not be persisted. As such, the results are
  shown in the variables view, with all the capabilities that allows,
  but also without the expression following the function as a watch
  expression would.
Debugger: Minor visual tweak in VariablesView.

- Expression evaluation results are now highlighted as changed when
  they're first added, since they're immediately of interest, unlike
  uninitialized variables that're first coming into scope.