ecere:Stacker; extras:SelectorBar: Further fixes to Stacker and SelectorBar addressin...

ecere:Stacker; extras:SelectorBar: Further fixes to Stacker and SelectorBar addressing memory leaks
* We needed to add a buttons holder to SelectorBar because the Stacker only keeps track of created controls
* Took out the 'delete child' from the Stacker's DestroyChildren(), because all the dereferencing that
was required to match the incref's within the Stacker was done by the controls.Free()
* These fixes are related to the problems http://ecere.com/mantis/view.php?id=656 and http://ecere.com/mantis/view.php?id=657
that led to a revert of the FlipStacker code: https://github.com/ecere/sdk/commit/28a771133cb578c040cf7f6e06588400eb3103ec