kcms/keys: make shortcut configuration keyboard-accessible

The list view that displays the shortcut-able actions for each category does not itself take keyboard focus, and neither do its (unexpanded) delegates. This means that unless the category only contains a single action (which will then be expanded and show the KeySequenceItem and possibly default shortcut checkbox), the keyboard shortcuts cannot be configured with the keyboard alone, as there is no way to select and unexpand an action delegate.

Instead, we allow the list view to take keyboard focus. To make the interaction clear, we set the first entry as the currentItem when the ListView receives focus, indicating that the user can now use arrow keys to select the desired action.