Fix crash caused by setting Tray window screen pointer on the QML side

Fix crash caused by setting Tray window screen pointer on the QML side

Seems like Qt sometimes doesn't like the QML window's screen property to be set
to a C++ (QVariant) pointer value, so we use the index: Qt.application.screens[]

See Qt docs: https://doc.qt.io/qt-5/qml-qtquick-window-window.html#screen-prop

This fix returns the matching window's index from the QGuiApplication::screens()
list to the QML side, instead of the window pointer.

Steps to reproduce the crash with the previous code:
- Open the Tray menu and close it a few times,
  or scroll randomly up and down in its activity list.

Tested with Qt 5.12.5

Signed-off-by: Michael Schuster <michael@schuster.ms>