Use wxPopupTransientWindow in wxComboCtrl in wxMSW too

This seems to work fine with current version of wxPopupTransientWindow
and wxPU_CONTAINS_CONTROLS style and allows to drastically simplify the
whole mess of preprocessor checks in wxComboCtrl code, reducing the
logic to just 2 cases: either wxUSE_POPUPWIN == 1 under any of the major
platforms and then we use wxPopupTransientWindow or wxUSE_POPUPWIN == 0
or we use some other platform and then a TLW-based fallback is used
instead.

In spite of the amount of changes, this should only modify behaviour
under MSW where wxPopupTransientWindow is used now instead of whatever
was used before.

include/wx/combo.h

@@ -726,9 +726,6 @@ private:
 
     wxByte                  m_ignoreEvtText;  // Number of next EVT_TEXTs to ignore
 
-    // Is popup window wxPopupTransientWindow, wxPopupWindow or wxDialog?
-    wxByte                  m_popupWinType;
-
     wxDECLARE_EVENT_TABLE();
 
     wxDECLARE_ABSTRACT_CLASS(wxComboCtrlBase);