QuickItemModel: Make more robust

Track QQuickItem::window changes properly. Untrack the QQI in case
QQI::window gets set to null.

This fixes a crash for me when inspecting a QQ2-based application that
creates/deletes QQIs on a regular basis.
QuickItemModel: Do not add items without a window

We do not want to display items that are not yet added to a scene.

This fixes another assert:
8  0x00007ffff784d5ba in qt_assert (assertion=0x7fffe15680d3
"sourceParent",
    file=0x7fffe1567ff8
"/home/krf/devel/src/gammaray/plugins/quickinspector/quickitemmodel.cpp",
line=249)
    at
/home/krf/devel/src/qt5/qtbase/src/corelib/global/qglobal.cpp:2096
9  0x00007fffe154f698 in GammaRay::QuickItemModel::itemReparented
(this=0xa969c0) at
/home/krf/devel/src/gammaray/plugins/quickinspector/quickitemmodel.cpp:249