Clear the currently selected value from a selection list in the plug-in configuration dialog

[Nicklas Nordborg]

The plug-in configuration dialog behaves a bit differently for enumerated parameters where it is possible to select multiple values versus where it is only possible to select a single value.

In the "select multiple" case there is an icon that allows the user to toggle between "select all" and "clear". In the "single option" case there is no corresponding icon for clearing the selected value.

The use case here is the "Annotation importer" that has automatic mapping between columns and annotation types, but sometimes the user only want to import a single column. It would be nice to be able to clear the mapping from unwanted matches by a single click instead of click+scroll+select.

[Nicklas Nordborg]

In 8318:

Fixes #2339: Clear the currently selected value from a selection list in the plug-in configuration dialog