set popupPanels centered. For Win the position is calculated.

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28066 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#3439 comment:8 - fix

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@28067 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e