Add tri-state checkboxes to the toplevel components

Maybe the IFW already supports this and this is actually a QTBUG - if so, please move.

The Qt installers have multiple toplevel components in which some subcomponents are installed by default. At the moment that is Qt Design Studio and some components under "Qt > Developer Tools". And more toplevel components might be created, spreading the default components around even more (QTPM-1029).

But there is no way to easily see which components are installed by default without expanding all toplevel nodes. I've now learned to expand and uncheck Design Studio first thing, but only after downloading and installing that multi-GB component a few times.

It would be very helpful to also have the tri-state checkboxes shown for toplevel components, so users can easily see where subcomponents are selected for installation without having to expand them all.