new patch for index DESC/NULLS FIRST/NULLS LAST

hi, all

new version Patch for TODO item:
- Add support for DESC/NULLS FIRST/NULLS LAST when creating indexes.

The Go method in dlgIndex seems clumsy. Maybe we need a new
pgIndexColumn class or another coding?

Now, OnAddCol, OnRemoveCol, GetColumns and OnChangeSize methods have
different vesion in dlgIndex and dlgIndexConstraint.

The OnChangeSize for Mac in dlgIndex rewrite to:
    lstColumns->SetSize(wxDefaultCoord, wxDefaultCoord,
        ev.GetSize().GetWidth(), ev.GetSize().GetHeight() - 800);
I don't know whether the size is right.

New method OnDescChange added to dlgIndex.

Please review.

-----------------------------------------------
Quan Zongliang
quanzongliang@gmail.com
CIT Japan:  http://www.cit.co.jp
CIT China:  http://www.citbj.com.cn