Fix wxShowEvent not being received for maximized frames on MSW

Send wxEVT_SHOW explicitly when showing a maximized TLW under MSW as the
system doesn't send WM_SHOW in this case (as documented in the MSDN and
also confirmed by testing).

Closes https://github.com/wxWidgets/wxWidgets/pull/1153

tests/toplevel/toplevel.cpp

@@ -24,6 +24,8 @@
     #include "wx/toplevel.h"
 #endif // WX_PRECOMP
 
+#include "testableframe.h"
+
 static void TopLevelWindowShowTest(wxTopLevelWindow* tlw)
 {
     CHECK(!tlw->IsShown());
@@ -75,3 +77,30 @@ TEST_CASE("wxTopLevel::Show", "[tlw][show]")
         frame->Destroy();
     }
 }
+
+// Check that we receive the expected event when showing the TLW.
+TEST_CASE("wxTopLevel::ShowEvent", "[tlw][show][event]")
+{
+    wxFrame* const frame = new wxFrame(NULL, wxID_ANY, "Maximized frame");
+
+    EventCounter countShow(frame, wxEVT_SHOW);
+
+    frame->Maximize();
+    frame->Show();
+
+    // Wait for the event to be received for the maximum of 2 seconds.
+    int showCount = 0;
+    for ( int i = 0; i < 40; i++ )
+    {
+        wxYield();
+
+        showCount = countShow.GetCount();
+
+        if ( showCount )
+            break;
+
+        wxMilliSleep(50);
+    }
+
+    CHECK( showCount == 1 );
+}