win32: add g_win32_get_command_line()

This returns the command line in GLib filename encoding format (ie:
UTF-8) for use with g_option_context_parse_strv().

This will allow parsing of Unicode commandline arguments on Windows,
even if the characters in those arguments fall outside of the range of
the system codepage.

https://bugzilla.gnome.org/show_bug.cgi?id=722025