* make sure after a showevent the size is final and the
dialog can be safely repositioned.
* set mainItem visible in :setVisible() so that is executed before showEvent:
resizing windows in their show event is definitely not enough, causes events to arrive to
reset to the old geometry in race with the setgeometry done there, don't know yet
if it's the qpa, qwindow, or the windowmanager
* make synctomaintem and updatelayoutparameters working even if
the dialog is not visible as we need to resize beforehand
* move the plasmasurface window also in the show event as if there was no moveevent
after an hide/show, its position would be resetted to 0,0