CORE-4079 Re-added default output folder validation.

Originally added in commit 2d1e447a0265f75a524e75794ab949d311207ca0.
Refactored defaultOpFolder initialization into a PreferencesViewImpl::initDefaultOutputFolder method.
Added an AbstractValidator to defaultOpFolder that checks for spaces in the output folder path.
Added a ValueChangeHandler to defaultOpFolder so that new values are validated when the user selects a different folder.
Added a defaultOpFolder::isValid check in the isValid method, which prevents the dialog from saving invalid folder paths.