Use wxWindowID in wxNewId() and related functions.

wxNewId(), wxRegisterId() and wxGetCurrentId() functions all work with
window IDs, so they should use the dedicated type. Previously, they
worked with long, which is not even the same type (wxWindowID is int),
causing implicit type conversion warnings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74485 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775