Provide support for storing PNG files on the clipboard (wxMSW)

PNG is a custom clipboard format but images stored in this format are supported by e.g. MS Excel, MS PowerPoint, GIMP (including image transparency).

See #17631.

include/wx/defs.h

@@ -2153,6 +2153,7 @@ enum wxDataFormatId
     wxDF_LOCALE =           16,
     wxDF_PRIVATE =          20,
     wxDF_HTML =             30, /* Note: does not correspond to CF_ constant */
+    wxDF_PNG =              31, /* Note: does not correspond to CF_ constant */
     wxDF_MAX
 };