Ticket #4071: checkbox centered (but not fully working yet)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31724 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#3985 opacity min is set to 0 

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31726 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e