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

docs/reference/glib/glib-sections.txt

@@ -1994,6 +1994,7 @@ g_completion_free
 <FILE>windows</FILE>
 MAXPATHLEN
 
+g_win32_get_command_line
 g_win32_error_message
 g_win32_getlocale
 g_win32_get_package_installation_directory