wxAui tab behaviour control

These changes allows the new wxAui to almost behave as the old one by default. To allow the new tab stacking behaviour you have now to catch the new wxEVT_AUI_PANE_CREATE_TAB event and do a Veto(false) in it. 

Using this event also allows to have a very fine grain control over tab creation that can be done dynamically pane by pane.

Try it, it works for me but need more testing, especially under Linux. The remaining differences are:
- in wxAuiNotebook it should create a tab, even when there is only one pane.
- A closing box is present next to tabs that I don't have with the original wxAui.

Regards,
Laurent