ContextualHelpButton: Fix things up

Fix up code style and qualified property access

Properties like hovered and toolTipVisible shouldn't be accessed through the parent property because in theory parent is just an Item.

Also make sure not to assume that root.parent always exists and provide a fallback value -1 which resets the Layout.maximumHeight.

Use a specific and appropriate type for a property

There are no consumers according to LXR search.

Sort out imports and improve docs

QtQuick.Window module is an alias to QtQuick, so it is redundant.