QbsProjectManager: Fix handling of overlapping parse requests.

The current code simply asserts when a new parse request comes in while
parsing. However, that condition is easily triggered, for instance if a
project file is saved to disk during a parse operation. Such updates
currently have no effect at all (other than triggering an error
message).
Instead, we now cancel the old parse job and start a new one.

Change-Id: If2eeb93b85b5163dcea99785a0fc89a254d082db
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>