Jump Lists: introduce "categories" & hide COM

QWinJumpList now has three built-in categories: Recent,
Frequent and Tasks. In addition to that, user can add
custom categories. A new type, QWinJumpListCategory,
represents all these categories, providing access to
items in the category.

The former COM-like begin(), append(), commit(),
abort() API has been replaced by a more property/
attribute-based API. The jump list automatically
invalidates and lazily rebuilds itself behind the
scenes. Furthermore, the API has been minimalized
for now - restoring the application identifier as
a full-fledged property is in the works.

Change-Id: I623a658b4b1fcfc881006f67e2300acadb483c97
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>