Factor out wxTopLevelWindow::MSWEnableCloseButton()

Make it possible to reuse this code for other, non-wx, windows.

No real changes, this is just a pure refactoring.

src/msw/toplevel.cpp

@@ -986,10 +986,11 @@ void wxTopLevelWindowMSW::SetIcons(const wxIconBundle& icons)
     DoSelectAndSetIcon(icons, SM_CXICON, SM_CYICON, ICON_BIG);
 }
 
-bool wxTopLevelWindowMSW::EnableCloseButton(bool enable)
+// static
+bool wxTopLevelWindowMSW::MSWEnableCloseButton(WXHWND hwnd, bool enable)
 {
     // get system (a.k.a. window) menu
-    HMENU hmenu = GetSystemMenu(GetHwnd(), FALSE /* get it */);
+    HMENU hmenu = GetSystemMenu(hwnd, FALSE /* get it */);
     if ( !hmenu )
     {
         // no system menu at all -- ok if we want to remove the close button
@@ -1008,7 +1009,7 @@ bool wxTopLevelWindowMSW::EnableCloseButton(bool enable)
         return false;
     }
     // update appearance immediately
-    if ( !::DrawMenuBar(GetHwnd()) )
+    if ( !::DrawMenuBar(hwnd) )
     {
         wxLogLastError(wxT("DrawMenuBar"));
     }
@@ -1016,6 +1017,11 @@ bool wxTopLevelWindowMSW::EnableCloseButton(bool enable)
     return true;
 }
 
+bool wxTopLevelWindowMSW::EnableCloseButton(bool enable)
+{
+    return MSWEnableCloseButton(GetHwnd(), enable);
+}
+
 // Window must have wxCAPTION and either wxCLOSE_BOX or wxSYSTEM_MENU for the
 // button to be visible. Also check for wxMAXIMIZE_BOX because we don't want
 // to enable a button that is excluded from the current style.