Add option NoCancelButtonOnLastPage to QWizard

Add support to hide the cancel button on the last page of a wizard. This
is useful for wizards where the last page is used as a summary page that
should not be cancelled.

[ChangeLog][QtWidgets][QWizard] Added NoCancelButtonOnLastPage option.

Task-number: QTBUG-7484
Change-Id: I282bda55a8dec9cde6439a9285d79e0a5c6df96a
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Optimize QObject::senderSignalIndex to only iterate the d->senders once.

QObject::senderSignalIndex is, combined with ::sender(), the current
hotspot in the QWebChannel which makes heavy use of it in its 1-to-many
QSignalSpy implementation.

This patch optimizes the senderSignalIndex by only iterating over the
linked senders list once, instead of twice. I.e. it first iterated over
it to find the signal index, then again to find the sender to convert
to a method-offset.

Change-Id: Ic86aed0dce891b87b953a6ec2364a81695bd4876
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>