Change "Item Layouts" to "Item Positioners"

This is because the title was changed in qtdeclarative.

Also add a line with link to Qt Quick Layouts

Change-Id: I044e53ae9c2cde39bd2b224caaf47017dff3455a
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>