Add Qt Quick Controls 2 support for dialogs

QtQuickControls1 does not handle embedded
platforms too well. In case of eglfs platform
we crash badly - only one window is supported.
QtQuickControls2 on the other hand lacks the native
look on desktop. Therefore on desktop platforms
keep using QtQuickControls1, and on eglfs use
QtQuickControls2.

QtQuick.Dialogs are not implemented for QtQuickControls2,
moreover required authentication dialog and prompt
dialog are neither implemented in QtQuickControls1.
As a workaround  make new dialogs to give
consistent look and feel.

Replace close() with reject() signal in java script
prompt dialog to unify handling between qqc1 and qqc2

[ChangeLog][QtWebEngine][General] Qt WebEngine (QML)
now optionally uses Qt Quick 2 Controls to show standard dialogs.

Task-number: QTBUG-53467
Task-number: QTBUG-51177
Change-Id: I42f9506357bbb82d4f04465f30a18c8013439e25
Reviewed-by: Kai Koehne <kai.koehne@qt.io>