Make default wxSizer border DPI-aware.

Scale the (still hard-coded) border in pixels by the content scale factor for
the platforms where this needs to be done, i.e. not wxGTK nor wxOSX where the
underlying toolkit already does it.

docs/changes.txt

@@ -123,6 +123,7 @@ wxGTK:
 
 wxMSW:
 
+- Make default wxSizer border DPI-aware.
 - Improve wxMimeTypesManager open command detection (Eric Jensen).
 - Make wxFILTER_INCLUDE_LIST in wxTextValidator actually usable.
 - Fix appearance of toggled wxToggleButtons with bitmap (tm).