Enable PopupWindows by default

Now that we have the popupType property in QQuickPopup, it would be nice to enable popup windows by default.

If the underlying platform can't use popup windows, it will fall back to popup items anyways. So might as well try to use popup windows as the default value.

Unfortunately, there are a lot of tests that will need to be rewritten, to take into account some of the changes that are in effect for popup windows.

Such as, waiting for the window to be active and polished, before sending events to the window.

In some cases, especially for Dialogs, the popup window will need to be the receiver of QTest::keyClick() and QTest::mouseClick() type events.

And tests that are checking the Overlay's children, won't find popups that use popup windows.

These are things to keep in mind when going trough the following list of tests:

tst_basic
tst_fusion
tst_imagine
tst_ios
tst_macos
tst_material
tst_windows

Ed:

tst_focus                                                               (2 failing tests: policy(ComboBox), reason())

Oliver:
tst_qquickmenu                                                (1 failing tests...)

 The tests above, are suspected to not work, since they've failed in the past, when we changed the default.
The exact test functions will need to be discovered again, as we go through each of those test classes.