Kit: Add constructor to restore Kit from QVariantMap

Do not trust kits with an invalid id, as there is no way those
could have ended up being saved out by creator. Safe all other kits
that were constructed using Kit(const QVariantMap &).

This new constructor uses the code that used to be in fromMap(...),
with some simplifications.

Remove fromMap(...) method from kit as it is no longer used.

Change-Id: Iac28ea9b85670e03088a4b7c5283af6b4b70c0fc
Reviewed-by: Daniel Teske <daniel.teske@digia.com>