GApplication: change commandline encoding policy

Clarify in the documentation that the commandline arguments passed
around by GApplication (to local_command_line and returned via
g_application_command_line_get_arguments()) are in the GLib filename
encoding (ie: UTF-8) on Windows, not the system code page.

Fix the mismatch that would result from having argv passed to
g_application_run() in main() on Windows (where it is in the system
code page) by ignoring argc/argv on Windows and calling
g_win32_get_command_line() for ourselves.  Document this.

This might be a slight API break on Windows: we documented that it was
possible to call g_application_run() with arguments other than argc/argv
and now doing that will result in those arguments being ignored.  It has
always been recommended practice to only call g_application_run() from
main() directly, however, and all of our code examples have shown only
this.  We will see if this causes any issues and consider reevaluating
the situation if so.

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

gio/gapplication.c

@@ -1505,15 +1505,18 @@ g_application_open (GApplication  *application,
  * is intended to be returned by main(). Although you are expected to pass
  * the @argc, @argv parameters from main() to this function, it is possible
  * to pass %NULL if @argv is not available or commandline handling is not
- * required.
+ * required.  Note that on Windows, @argc and @argv are ignored, and
+ * g_win32_get_command_line() is called internally (for proper support
+ * of Unicode commandline arguments).
  *
  * First, the local_command_line() virtual function is invoked.
  * This function always runs on the local instance. It gets passed a pointer
- * to a %NULL-terminated copy of @argv and is expected to remove the arguments
- * that it handled (shifting up remaining arguments). See
- * <xref linkend="gapplication-example-cmdline2"/> for an example of
- * parsing @argv manually. Alternatively, you may use the #GOptionContext API,
- * after setting <literal>argc = g_strv_length (argv);</literal>.
+ * to a %NULL-terminated copy of the command line and is expected to
+ * remove the arguments that it handled (shifting up remaining
+ * arguments). See <xref linkend="gapplication-example-cmdline2"/> for
+ * an example of parsing @argv manually. Alternatively, you may use the
+ * #GOptionContext API, but you must use g_option_context_parse_strv()
+ * in order to avoid memory leaks and encoding mismatches.
  *
  * The last argument to local_command_line() is a pointer to the @status
  * variable which can used to set the exit status that is returned from
@@ -1604,16 +1607,23 @@ g_application_run (GApplication  *application,
 {
   gchar **arguments;
   int status;
-  gint i;
 
   g_return_val_if_fail (G_IS_APPLICATION (application), 1);
   g_return_val_if_fail (argc == 0 || argv != NULL, 1);
   g_return_val_if_fail (!application->priv->must_quit_now, 1);
 
-  arguments = g_new (gchar *, argc + 1);
-  for (i = 0; i < argc; i++)
-    arguments[i] = g_strdup (argv[i]);
-  arguments[i] = NULL;
+#ifdef G_OS_WIN32
+  arguments = g_win32_get_command_line ();
+#else
+  {
+    gint i;
+
+    arguments = g_new (gchar *, argc + 1);
+    for (i = 0; i < argc; i++)
+      arguments[i] = g_strdup (argv[i]);
+    arguments[i] = NULL;
+  }
+#endif
 
   if (g_get_prgname () == NULL)
     {

gio/gapplicationcommandline.c

@@ -348,7 +348,12 @@ g_application_command_line_class_init (GApplicationCommandLineClass *class)
  *
  * Gets the list of arguments that was passed on the command line.
  *
- * The strings in the array may contain non-utf8 data.
+ * The strings in the array may contain non-UTF-8 data on UNIX (such as
+ * filenames or arguments given in the system locale) but are always in
+ * UTF-8 on Windows.
+ *
+ * If you wish to use the return value with #GOptionContext, you must
+ * use g_option_context_parse_strv().
  *
  * The return value is %NULL-terminated and should be freed using
  * g_strfreev().