LCL: PageControl, changing (moving) the tab position of a page, would sometimes (but not always) trigger OnChanging. OnChanging could prevent the change, while it must always be allowed during moving tabs.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42682 4005530d-fff6-0310-9dd1-cebe43e6787f