Possibility to to "disable" top level check box in install-tree (but still allow selecting child nodes)

In the installers install-tree allow a possibility to disable selecting a node, but child nodes could still be selected as normal. E.g. define this in components package.xml something like:

<AllowSelection>false</AllowSelection> <!-- deafult is True-->

The rationale behind this:

Especially for embedded offering, people often just select the top level item in install tree thus they end up installing all possible embedded packages ~10-20G accidentally. This causes installation cancellations and unnecessary network traffic.

If we could disable clicking on e.g. qt.embedded.b2qt [] check box the user would need to actually "think" what he needs to install under the sub-components.