kcm: conform better to KDE HIG
- For the "Add user" button, use more universal icon and a shorter, more contextually-driven text.
- For the "Delete user" text, use a universal icon rather than a specific one, since the context makes the target obvious.
- Use alternating row background colors for list items to help the user line up the items with their action buttons on the other side.
- Add explanation to placeholder message.
How the list view will look now:
