Don't reset the moved display position

This fixes the problem that after moving a display, clicking on another one
and clicking on the original one, its position got reset to the default one.
This due to layouting in the widget when displays are clicked, the
ControlPanel hides/shows the corresponding OutputConfig, which results in a
temporary layout changed, that causes a geometry change for the QML view,
that causes am updateOutpusPlacement()...