Fix bug with wxRadioButton state changing unexpectedly in wxMSW

In wxMSW, a focused wxRadioButton is always checked, which meant that
checking a wxRadioButton while focus was not in the window containing it
and later giving the focus to that window could uncheck it by giving
focus to another wxRadioButton that had had it previously.

Fix this by adding WXSetPendingFocus() to wxMSW wxWindow and calling it
from wxRadioButton::SetValue() to ensure that when the focus is
regained, it goes to the newly checked radio button and not some other
one.

This replaces the previously used, for the same purpose, wxMSW-specific
wxTopLevelWindow::SetLastFocus(), so while this solution is not exactly
pretty, it's not worse than we had before, while being more generic.

Also add a unit test checking that things work correctly in the scenario
described above.

Closes https://github.com/wxWidgets/wxWidgets/pull/1257

Closes #18341.

include/wx/msw/toplevel.h

@@ -100,9 +100,12 @@ public:
     // event handlers
     void OnActivate(wxActivateEvent& event);
 
-    // called by wxWindow whenever it gets focus
-    void SetLastFocus(wxWindow *win) { m_winLastFocused = win; }
-    wxWindow *GetLastFocus() const { return m_winLastFocused; }
+    // called from wxWidgets code itself only when the pending focus, i.e. the
+    // element which should get focus when this TLW is activated again, changes
+    virtual void WXDoUpdatePendingFocus(wxWindow* win) wxOVERRIDE
+    {
+        m_winLastFocused = win;
+    }
 
     // translate wxWidgets flags to Windows ones
     virtual WXDWORD MSWGetStyle(long flags, WXDWORD *exstyle) const wxOVERRIDE;