Merge branch 'border-size-functions'

Replace DoGetBorderSize() with GetWindowBorderSize() to avoid having 2
different functions doing (almost) the same thing.

See https://github.com/wxWidgets/wxWidgets/pull/2445

docs/changes.txt

@@ -196,6 +196,9 @@ Changes in behaviour which may result in build errors
 - wxImage constructor from XPM data is now explicit, write "wxImage(xpmData)"
   instead of just "xpmData" if you really need to use it.
 
+- wxWindow::DoGetBorderSize() was removed, if you used this non-public function
+  in your code, you can replace it with public GetWindowBorderSize().
+
 - Microsoft Visual Studio 2003 (a.k.a. MSVC 7) is not supported any longer, the
   minimum required version is now 2005.