CodeAssit: Fix completion if function hint is shown

This reverts commit c5dfcce948dd40dac4f1a22d97a53afc5c6b7494, but the
bug addressed by that commit remains fixed. FunctionHintProposals need
to be fragile so they can be replaced by e.g. GenericProposals. Only one
proposal can be "active".

Change-Id: Ibca40063d46b8ef3bce4d72be7488f70885e75da
Reviewed-by: David Schulz <david.schulz@digia.com>
ProjectWindow: Do not listen for Kit updates during shutdown

The SessionManager is disconnected at that point already, so we were
never told that projects were destructed already and were happily
calling into those.

This fixes the crash described in QTCREATORBUG-10354.

Task-number: QTCREATORBUG-10354
Change-Id: Ib8ce07d6db38dbff9154b127f5fb620a9961c742
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>