Fixed several bugs in wxDateTime timezone handling:

 - ToTimezone() and MakeTimezone() now work as expected
 - added and documented FromTimezone() and MakeFromTimezone()
 - Set(double jdn) interprets jdn always in UTC
 - updated ParseRfc822Date() timezone handling
 - removed workarounds for old bugs from the test

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35334 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775