Provide singletons as an alternative to Qt.<someObject> properties

In Qt 6.2, we made Application available as a singleton. There are however further types which deserve the same treatment:

• platform,
• inputMethod and
• styleHints

At least the variants provided by the QtQuick module should be exposed as singletons. This will provide better type information, and avoids the "this property only exists on the object if Quick has been imported" issue.