Add onSelect method for treeCtrl to set folderIdx
Add onSelect method for treeCtrl to set folderIdx

folderIdx is part of rootScope and can be referenced later when
creating new objects. This way we can keep track of which folder is
selected.

stardust66/math3d,stardust66/math3d,stardust66/math3d,ChristopherChudzicki/math3d,ChristopherChudzicki/math3d,ChristopherChudzicki/math3d,stardust66/math3d,ChristopherChudzicki/math3d