lazcontrols: lvlgraph: fixed singleselect, added property Selected

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40609 4005530d-fff6-0310-9dd1-cebe43e6787f
IDE: package graph: using lvlgraph

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40610 4005530d-fff6-0310-9dd1-cebe43e6787f