Fix handling TAB in wxSearchCtrl in wxMSW

Check whether wxTextCtrl has focus using Win32 ::GetFocus() instead of
by comparing FindFocus() with "this" pointer, as the latter fails when
wxTextCtrl is part of a composite control, such as wxSearchCtrl.

This is enough to make pressing TAB in wxSearchCtrl correctly go to the
next control instead of just beeping.

Note that we could also use DoFindFocus() here, but this would be
needlessly less efficient as we don't really need to find wxWindow
corresponding to the focused HWND, as that function does. But we can't
use HasFocus() here as it wouldn't work correctly here when the focus is
on another sub-window of a composite control also containing this
wxTextCtrl.

src/msw/textctrl.cpp

@@ -2078,7 +2078,7 @@ void wxTextCtrl::OnChar(wxKeyEvent& event)
             // forces at the moment unfortunately
             if ( !(m_windowStyle & wxTE_PROCESS_TAB))
             {
-                if ( FindFocus() == this )
+                if ( ::GetFocus() == GetHwnd() )
                 {
                     int flags = 0;
                     if (!event.ShiftDown())