Implement monitoring of file descriptors in wxMotif event loop.

This allows applications using wxMotif to link again after the changes of
r74350 -- and wxExecute() unit tests actually pass, too.

Closes #15305.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74480 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775