Add a private property to ASMake tasks.

This can be used to flag tasks that are not meant
to be run directly. Such tasks are not registered
in the `tasks` list.

Note that, for a private task, the private property
must be set *before* the parent property.
Add name, version, and id properties to ASMake.
Support top-level name property and aliases in ASMake.

Also remove support for the private property,
because after the refactoring it cannot be used
any more.
Add synonyms for help and version tasks.