Bump WebKitGTK+ requirements to 2.7.2

We will use the new API added in 2.7.2 like script messages.
Use user a script message instead of a DBus signal for RemoveItemFromOverview
Use user a script message instead of a DBus signal for AllowTLSCertificate
Use user a script message instead of a DBus signal for FormAuthDataSaveConfirmationRequired