QbsProjectManager: Make Kit function argument const.

It's silly to force all callers in the chain to use a non-const pointer
for a Kit that is really const just because of the QList<T *> vs.
QList<const T *> problem. Instead, use a (safe) const_cast for the
list operations.

Change-Id: I929cb9e42957de48d7ad8e1e4b04a7739f9c22bf
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
QbsProjectManager: Make QbsManager instance available as a Singleton.

We now use it in contexts that do not have access to a Project.

Change-Id: I96ebad60f5fc354b004092748033d83c766a305d
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>