Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

New in clang 3.6.

Move code from Path::RelativeFS() and make it generic.

There is no GetLengthUTF8() because strlen or std::string::size()
could be used instead.

- Move definition of SEPARATOR_UTF8 out of #ifdef
- Remove duplicated check in IsSeparatorUTF8

Remove a GLib dependencies from class EventLoop and DatabaseLock.