Use flat generic status bar by default and add wxSB_SUNKEN.

GTK+ applications don't use sunken status bars since many years, do don't do
it in wxWidgets neither by default any more. Add wxSB_SUNKEN style that can be
explicitly used if the old appearance is desired.

Closes #15009.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73691 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/statusbar.cpp

@@ -275,6 +275,7 @@ void wxStatusBar::DoUpdateStatusText(int nField)
         style = SBT_NOBORDERS;
         break;
 
+    case wxSB_SUNKEN:
     case wxSB_NORMAL:
     default:
         style = 0;
@@ -553,6 +554,7 @@ void wxStatusBar::SetStatusStyles(int n, const int styles[])
         case wxSB_FLAT:
             style = SBT_NOBORDERS;
             break;
+        case wxSB_SUNKEN:
         case wxSB_NORMAL:
         default:
             style = 0;