QQuickTooltip should not crash if widget implementation isn't possible

It's not enough to check for QT_NO_WIDGETS because it can fail at
runtime if the app was launched with a QGuiApplication or you are on
a platform without multiple-window support.

Task-number: QTBUG-32779
Change-Id: I13abf9a5063f7d91277bc52b8be79bee19e746af
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>