Small optimization of wxFFile::ReadAll(): avoid extra string copy.

Small optimization of wxFFile::ReadAll(): avoid extra string copy.

Use swap() to move the newly created string into its destination instead of
copying it there. This can be relatively important as the string represents an
entire file contents here and so could be quite long.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72597 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775