make setting default Kit work

Copying from working copy to the real kit copy causes KM to reset
changes and call "discard()". Then m_isDefaultKit is reset which makes
the logic in "apply()" a mess. Try to avoid this by saving
m_isDefaultKit

Task-number: QTCREATORBUG-8205
Change-Id: Ie8243d7dc2aca92788d719c0c147127b3a7e8714
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>