Add wxPU_CONTAINS_CONTROLS style for wxPopupWindow

This restores the default behavior of a popup window in MSW to the
behavior it had before 56c4191168. The new
flag added by this commit can be used to give the popup window the
behavior from after that commit, i.e. choose the implementation using a
WS_POPUP window rather than the default one using a WS_CHILD of the
desktop.

The old behavior kept the popup from taking focus from its parent window
but left some controls not working. The new behavior has the popup take
focus and lets all controls work.

Closes https://github.com/wxWidgets/wxWidgets/pull/1123

include/wx/msw/popupwin.h

@@ -25,11 +25,15 @@ public:
 
     bool Create(wxWindow *parent, int flags = wxBORDER_NONE);
 
+    virtual void SetFocus() wxOVERRIDE;
     virtual bool Show(bool show = true) wxOVERRIDE;
 
     // return the style to be used for the popup windows
     virtual WXDWORD MSWGetStyle(long flags, WXDWORD *exstyle) const wxOVERRIDE;
 
+    // get the HWND to be used as parent of this window with CreateWindow()
+    virtual WXHWND MSWGetParent() const wxOVERRIDE;
+
 
     // Implementation only from now on.