[BLUEJ-925] CodePad needs indicator that variables are no longer valid after re-compile

In previous versions of BlueJ, when a class was recompiled the CodePad would clear out, providing an indication that variables were no longer valid.

In version 4.0.0, the CodePad no longer blanks out, misleading the user into thinking the variables are still available. There needs to be some visual indication, such as a line, to show that variables have been reset.

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