Fix bitmap position attribute name in XRC handler.

Fix bitmap position attribute name in XRC handler.

Bitmap position attribute was called "bitmapposition" in the documentation and
in the code that generated an error message for a wrong value in it but was
inexplicably called "direction" in the code that really looked it up.

This seems to be just a straight typo from the original r61065. Surprising as
it is that it wasn't noticed before, do correct it now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68680 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775