JBIDE-13602 : instanciate a Stacks in all circumstances

egit messed up the line ending, I only managed to recover the New...Page for proper diff.
As for StacksManager, the interesting diff is at https://github.com/fbricon/jbosstools-central/pull/new/jbosstools:jbosstools-4.0.x...JBIDE-13602#L0R57