widget should position screen center when add/remove screen

When using two screens, plug and unplug the screen, monitor the QGuiApplication::screenAdded and QGuiApplication::screenRemoved signals, and set the current widget to center display

Phenomenon: When there are two screens, the widget is not displayed in the center

the geometry.zip can reproduce this bug.

when run the demo, the widget show center see 1.png,

when remove a screen and wait a scecond, the widget not show the center see 2.png

note: under the debian10(Qt version is 5.15.3), this bug always exist