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/generic/statusbr.cpp

@@ -291,10 +291,10 @@ void wxStatusBarGeneric::DrawField(wxDC& dc, int i, int textHeight)
         return;     // happens when the status bar is shrunk in a very small area!
 
     int style = m_panes[i].GetStyle();
-    if (style != wxSB_FLAT)
+    if (style == wxSB_RAISED || style == wxSB_SUNKEN)
     {
         // Draw border
-        // For wxSB_NORMAL: paint a grey background, plus 3-d border (one black rectangle)
+        // For wxSB_SUNKEN: paint a grey background, plus 3-d border (one black rectangle)
         // Inside this, left and top sides (dark grey). Bottom and right (white).
         // Reverse it for wxSB_RAISED