Use flat generic status bar by default and add wxSB_SUNKEN.

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