Add MSWGetFocusHWND to allow focus a subwindow

Under MSW allow override which subwindow will be focused for composite
windows which are implemented not as a set of wxControl (i.e. using only
Windows native controls).

include/wx/msw/private.h

@@ -838,6 +838,7 @@ private:
 // ---------------------------------------------------------------------------
 
 #define GetHwnd()               ((HWND)GetHWND())
+#define MSWGetFocusHwnd()       ((HWND)MSWGetFocusHWND())
 #define GetHwndOf(win)          ((HWND)((win)->GetHWND()))
 // old name
 #define GetWinHwnd              GetHwndOf

include/wx/msw/window.h

@@ -168,6 +168,11 @@ public:
     void AssociateHandle(WXWidget handle) wxOVERRIDE;
     void DissociateHandle() wxOVERRIDE;
 
+    // returns the handle of the focused window (i.e. in composite windows
+    // which can't be implemented via a set of wxControl's focus may be wanted
+    // by a subwindow instead of the main window)
+    virtual WXHWND MSWGetFocusHWND() const { return GetHWND(); }
+
     // does this window have deferred position and/or size?
     bool IsSizeDeferred() const;
 

src/msw/window.cpp

@@ -577,7 +577,7 @@ void wxWindowMSW::SetId(wxWindowID winid)
 
 void wxWindowMSW::SetFocus()
 {
-    HWND hWnd = GetHwnd();
+    HWND hWnd = MSWGetFocusHwnd();
     wxCHECK_RET( hWnd, wxT("can't set focus to invalid window") );
 
     ::SetLastError(0);
@@ -599,13 +599,15 @@ void wxWindowMSW::SetFocus()
 
 void wxWindowMSW::SetFocusFromKbd()
 {
+    HWND hWnd = MSWGetFocusHwnd();
+
     // when the focus is given to the control with DLGC_HASSETSEL style from
     // keyboard its contents should be entirely selected: this is what
     // ::IsDialogMessage() does and so we should do it as well to provide the
     // same LNF as the native programs
-    if ( ::SendMessage(GetHwnd(), WM_GETDLGCODE, 0, 0) & DLGC_HASSETSEL )
+    if ( ::SendMessage(hWnd, WM_GETDLGCODE, 0, 0) & DLGC_HASSETSEL )
     {
-        ::SendMessage(GetHwnd(), EM_SETSEL, 0, -1);
+        ::SendMessage(hWnd, EM_SETSEL, 0, -1);
     }
 
     // do this after (maybe) setting the selection as like this when