Simplify changing window styles in wxMSW code

Add wxMSWWinStyleUpdater and wxMSWWinExStyleUpdater helper classes which
allow writing code changing GWL_STYLE and GWL_EXSTYLE bits,
respectively, in a shorter and more clear way.

There should be no real changes in behaviour.

src/msw/statbox.cpp

@@ -45,6 +45,7 @@
 #include "wx/msw/private.h"
 #include "wx/msw/missing.h"
 #include "wx/msw/dc.h"
+#include "wx/msw/private/winstyle.h"
 
 // the values coincide with those in tmschema.h
 #define BP_GROUPBOX 4
@@ -303,10 +304,10 @@ WXHRGN wxStaticBox::MSWGetRegionWithoutChildren()
             continue;
         }
 
-        LONG style = ::GetWindowLong(child, GWL_STYLE);
+        wxMSWWinStyleUpdater updateStyle(child);
         wxString str(wxGetWindowClass(child));
         str.UpperCase();
-        if ( str == wxT("BUTTON") && (style & BS_GROUPBOX) == BS_GROUPBOX )
+        if ( str == wxT("BUTTON") && updateStyle.IsOn(BS_GROUPBOX) )
         {
             if ( child == GetHwnd() )
                 foundThis = true;
@@ -329,10 +330,9 @@ WXHRGN wxStaticBox::MSWGetRegionWithoutChildren()
         {
             // need to remove WS_CLIPSIBLINGS from all sibling windows
             // that are within this staticbox if set
-            if ( style & WS_CLIPSIBLINGS )
+            if ( updateStyle.IsOn(WS_CLIPSIBLINGS) )
             {
-                style &= ~WS_CLIPSIBLINGS;
-                ::SetWindowLong(child, GWL_STYLE, style);
+                updateStyle.TurnOff(WS_CLIPSIBLINGS).Apply();
 
                 // MSDN: "If you have changed certain window data using
                 // SetWindowLong, you must call SetWindowPos to have the