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

include/wx/statusbr.h

@@ -45,6 +45,7 @@ extern WXDLLIMPEXP_DATA_CORE(const char) wxStatusBarNameStr[];
 #define wxSB_NORMAL    0x0000
 #define wxSB_FLAT      0x0001
 #define wxSB_RAISED    0x0002
+#define wxSB_SUNKEN    0x0003
 
 // ----------------------------------------------------------------------------
 // wxStatusBarPane: an helper for wxStatusBar
@@ -150,10 +151,7 @@ public:
     // field styles
     // ------------
 
-    // Set the field style. Use either wxSB_NORMAL (default) for a standard 3D
-    // border around a field, wxSB_FLAT for no border around a field, so that it
-    // appears flat or wxSB_POPOUT to make the field appear raised.
-    // Setting field styles only works on wxMSW
+    // Set the field border style to one of wxSB_XXX values.
     virtual void SetStatusStyles(int n, const int styles[]);
 
     int GetStatusStyle(int n) const