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

interface/wx/statusbr.h

@@ -20,6 +20,7 @@
 #define wxSB_NORMAL    0x0000
 #define wxSB_FLAT      0x0001
 #define wxSB_RAISED    0x0002
+#define wxSB_SUNKEN    0x0003
 
 
 /**
@@ -268,10 +269,12 @@ public:
             number passed to SetFieldsCount() the last time it was called.
         @param styles
             Contains an array of @a n integers with the styles for each field.
-            There are three possible styles:
-            - @c wxSB_NORMAL (default): The field appears sunken with a standard 3D border.
+            There are four possible styles:
+            - @c wxSB_NORMAL (default): The field appears with the default native border.
             - @c wxSB_FLAT: No border is painted around the field so that it appears flat.
             - @c wxSB_RAISED: A raised 3D border is painted around the field.
+            - @c wxSB_SUNKEN: A sunken 3D border is painted around the field
+              (this style is new since wxWidgets 2.9.5).
     */
     virtual void SetStatusStyles(int n, const int* styles);