The 'Test with file' function doesn't clear values from parent form

[Nicklas Nordborg]

1. Enter a plug-in configuration sequence for one of the flat file parser plug-ins, for example the reporter importer
2. Enter a value for some of the regular expressions, for example, the ignore expression
3. Use the "Test with file" function and clear the corresponding value in the new popup
4. Click ok to close the "Test with file" form
5. The value is still present in the main configuration form. The 'x' mark may be gone, but as soon as you click on the parameter the original value magically reappears.

[Nicklas Nordborg]

(In [3566]) Fixes #678, #679, #680