Provide keyboard shortcuts in (de-)selection and (de-)filter dialogs

Provide keyboard shortcuts for "Apply selection to folders" in (de-)selection dialog, and for "Apply filter to folders" in filter dialog. Other related minor changes.

FIXED: [ 411446 ] Provide keyboard shortcut for "apply selection to folders" in (de-)selection dialog
BUG: https://bugs.kde.org/show_bug.cgi?id=411446

Differential Revision: https://phabricator.kde.org/D23606