Enhance the user experience of QWindowCapture with QWindow/qml Window/WinId

A user may have a native window, QWindow, or QML Window, and it can be problematic to map it to QCapturableWindow. To improve the user flow, we want to have something like a named constructor QCaptureWindow::fromWindow(QWindow/qml window) that may create a valid or invalid capturable window depending on the input.

The documentation of QWindowCapture::window should also be improved in the scope of this user story.