window: fix modified forms check when closing single tab

This regressed in e14780a2. I tested this commit extensively with a broken web process to ensure it's possible to close tabs with a misbehaving web process, but failed to test with a well-behaved web process. The tab now closes after the timeout, while the modified forms dialog is being shown, without waiting for user input. Oops. Got to cancel the timeout once we've received our response from the web process.