Get a frameless, always on top preview at a given position, like in the old picamera

[JamesRayn]

Hi, it would be greate to get the availability to create a frameless, always on top preview at a given position, like in the old picamera.

In my case I simply added (after importing QtCore)

    self.qpicamera2.setWindowFlags(QtCore.Qt.FramelessWindowHint | QtCore.Qt.WindowStaysOnTopHint)
    self.qpicamera2.move(100, 100)

between L14 and L15.

picamera2/picamera2/previews/qt_gl_preview.py

Lines 14 to 15 in adaec85

	self.qpicamera2.setWindowTitle("QtGlPreview")
	self.qpicamera2.show()

May be you could add this to the public code as a optional parameter.

[ventilator]

This would be very useful for applications written in other UI frameworks (e.g. tk) which are using the camera preview, e.g. a photobox with fullscreen UI controls in the background beneath the camera preview