Avoid positioning the panel when it has no size yet

Calling setPosition with a position of the left-most pixel of a screen
and  size of 0x0 will set the screen to be the one to the left of it.
Making screenToFollow() and QWindow::screen() out of sync temporarily.
While arguably a Qt issue and this being fixed up later when the panel
resizes and other properties are updated, we need to workaround this
to our usage of layer-shell. Layer surfaces can't change screen once
created. It can happen that the surface is created during the time
that the panel is on the wrong screen, resulting in the panel
appearing on the wrong screen while thinking it is on another one.
To fix this the initial positionPanel() call is removed, the
panel will now be positioned after it has resized from the event handler.
BUG:483102
BUG:483348
BUG:470434
FIXED-IN:6.0.3


(cherry picked from commit 27dd6897)