Allow a pane to always dock in a notebook

This change set mainly allows a pane to always dock in notebook, even if it is not stacked with another pane.
It introduces the optionAlwaysDockInNotebook pane option and associated Set/Get.
A change is also made to the name (wxEVT_AUI_PANE_DOCK_OVER) of the event that allow panes to stack: it no more talks of "Tabs" but rather of "Docking over" which is more consistent with the MoveOver() method and the fact we may have isolated tabs.
The wxAuiManagerEvent gains Allow(), IsAllowed() methods to be in par with the wxNotifyEvent.