Designer: minor refactoring, remove PersistentIsMarkedForDeletion method.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46071 4005530d-fff6-0310-9dd1-cebe43e6787f
LCL + Designer: A new flag TWinControl.DesignerDeleting. Make TPairSplitter use it. Issue #26618

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46072 4005530d-fff6-0310-9dd1-cebe43e6787f
Designer: remove a hack IgnoreDeletingPersistent. It was replaced by another hack TWinControl.DesignerDeleting.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46073 4005530d-fff6-0310-9dd1-cebe43e6787f