Handle ViewHostMsg_Focus message from chromium

Calling the method window.focus() in javascript causes chromium to send
a ViewHostMsg_Focus message, which is received by RenderViewHost,
which then calls RenderViewHost::OnFocus.
This calls WebContentsDelegate::ActivateContents, which does
nothing in the default implementation. We now override this method in
WebContentsDelegateQt::ActivateContents and call Focus() on
the WebContents object IF the new WebEngineSettings value
AllowWindowActivationFromJavaScript is true (by default, it is false).
This in turn calls QWebEnginePagePrivate::focusContainer.
The WebEnginePage now calls QWidget::activateWindow() in addition
to QWidget::setFocus, to make sure the window is also activated (which
it might not be, if multiple windows are open).
For the QML side, a new boolean Property
allowWindowActivationFromJavaScript was added.

Task-number: QTBUG-58800
Change-Id: Iabf5d4d15236c77838a3886de81e9dafcaf49f5d
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>