Added wxGetUTCTimeMillis() and wxGetUTCTimeUSec().

Added wxGetUTCTimeMillis() and wxGetUTCTimeUSec().

Fixed wxGetLocalTimeMillis() to really return the local time and provide
wxGetUTCTimeMillis() doing what this function used to do before. Closes #13610.

Also add wxGetUTCTimeUSec() for even higher resolution time stamps as it is
basically the same as wxGetUTCTimeMillis() anyhow, at least for MSW and
non-ancient Unix systems providing gettimeofday().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69837 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775