Add wxLoadUserResource() overload not copying the resource data.

Add wxLoadUserResource() overload not copying the resource data.

The existing wxLoadUserResource() copies the resource data which is often
unnecessary. Add another overload which just returns the pointer directly to
the resource data.

Also move the function into base from core as it can be useful for the console
applications as well.

Finally, define wxUserResourceStr used by this function only in the same file
where the function itself is defined instead of datacmn.cpp.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64150 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775