Fix regression in wxTopLevelWindowMSW::IsMaximized()

This fixes another bug from 3518f1a7d8
(after the one fixed in 5766280311): if a
window was maximized by user and then hidden, its IsMaximized() returned
false because it examined m_showCmd which didn't have SW_MAXIMIZE value
in this case.

The same was true for IsMinimised() as well.

Fix both problems by updating the value of m_showCmd when hiding the
window.

src/msw/toplevel.cpp

@@ -579,6 +579,15 @@ bool wxTopLevelWindowMSW::Show(bool show)
     }
     else // hide
     {
+        // When hiding the window, remember if it was maximized or iconized in
+        // order to return the correct value from Is{Maximized,Iconized}().
+        if ( ::IsZoomed(GetHwnd()) )
+            m_showCmd = SW_MAXIMIZE;
+        else if ( ::IsIconic(GetHwnd()) )
+            m_showCmd = SW_MINIMIZE;
+        else
+            m_showCmd = SW_SHOW;
+
         nShowCmd = SW_HIDE;
     }