Explicitly mention that wxDateTime ticks origin is in UTC.

Make it clear that the number of ticks is counted since the same moment in all
time zones.

See #13098.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74422 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix FILETIME <-> wxDateTime conversions while DST is in effect in wxMSW.

The result was (consistently, so the tests still passed) off by an hour when
the program was ran while DST was in effect. Fix this by avoiding the use of
FileTimeToLocalFileTime() and LocalFileTimeToFileTime() and just directly
converting FILETIME values to wxDateTime. Not only this is more correct but
it's also simpler and more efficient as well.

Also add a unit test for wxFileName::SetTimes() too.

Closes #13098.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74423 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775