Bug-790839 GApplication command line --help enhancements

In order to enrich information displayed by GApplication command line
handling when --help is invoked, 3 new methods are proposed:
. g_application_set_option_context_parameter_string
. g_application_set_option_context_summary
. g_application_set_option_context_description
Those methods interact with the GApplication's internal GOptionContext
which is created for command line parsing in g_application_parse_command_line.
(please refer to the GOptionContext class for more information about option
context, parameter string, summary and description.)

To illustrate the 3 methods, an example is provided:
. gapplication-example-cmdline4.c

gio/tests/Makefile.am

@@ -87,6 +87,7 @@ uninstalled_test_extra_programs = \
 	gapplication-example-cmdline		\
 	gapplication-example-cmdline2		\
 	gapplication-example-cmdline3		\
+	gapplication-example-cmdline4		\
 	gapplication-example-dbushooks		\
 	gapplication-example-open		\
 	gdbus-example-export			\

gio/tests/gapplication-example-cmdline4.c

@@ -0,0 +1,85 @@
+#include <gio/gio.h>
+#include <stdlib.h>
+#include <string.h>
+
+
+static gint
+handle_local_options (GApplication      *application,
+                      GVariantDict      *options,
+                      gpointer           user_data)
+{
+  guint32 count;
+
+  /* Deal (locally) with version option */
+  if (g_variant_dict_lookup (options, "version", "b", &count))
+    {
+      g_print ("This is example-cmdline4, version 1.2.3\n");
+      return EXIT_SUCCESS;
+    }
+
+  return -1;
+
+}
+
+static gint
+command_line (GApplication                *application,
+              GApplicationCommandLine     *cmdline,
+              gpointer                     user_data)
+{
+  guint32 count;
+
+  GVariantDict *options = g_application_command_line_get_options_dict (cmdline);
+
+  /* Deal with arg option */
+  if (g_variant_dict_lookup (options, "flag", "b", &count))
+    {
+      g_application_command_line_print (cmdline, "flag is set\n");
+    }
+
+  return EXIT_SUCCESS;
+}
+
+
+int
+main (int argc, char **argv)
+{
+  GApplication *app;
+  int status;
+
+  GOptionEntry entries[] = {
+    /* A version flag option, to be handled locally */
+    { "version", 'v', G_OPTION_FLAG_NONE, G_OPTION_ARG_NONE, NULL, "Show the application version", NULL },
+
+    /* A dummy flag option, to be handled in primary */
+    { "flag", 'f', G_OPTION_FLAG_NONE, G_OPTION_ARG_NONE, NULL, "A flag argument", NULL },
+
+    { NULL }
+  };
+
+  app = g_application_new ("org.gtk.TestApplication",
+                           G_APPLICATION_HANDLES_COMMAND_LINE);
+
+  g_application_add_main_option_entries (app, entries);
+
+  g_application_set_option_context_parameter_string (app, "- a simple command line example");
+  g_application_set_option_context_summary (app,
+                                            "Summary:\n"
+                                            "This is a simple command line --help example.");
+  g_application_set_option_context_description (app,
+                                                "Description:\n"
+                                                "This example illustrates the use of "
+                                                "g_application command line --help functionalities "
+                                                "(parameter string, summary, description). "
+                                                "It does nothing at all except displaying information "
+                                                "when invoked with --help argument...\n");
+
+  g_signal_connect (app, "handle-local-options", G_CALLBACK (handle_local_options), NULL);
+  g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);
+
+  /* This application does absolutely nothing, except if a command line is given */
+  status = g_application_run (app, argc, argv);
+
+  g_object_unref (app);
+
+  return status;
+}

gio/tests/meson.build

@@ -309,6 +309,7 @@ uninstalled_test_extra_programs = [
   ['gapplication-example-cmdline'],
   ['gapplication-example-cmdline2'],
   ['gapplication-example-cmdline3'],
+  ['gapplication-example-cmdline4'],
   ['gapplication-example-dbushooks'],
   ['gapplication-example-open'],
   ['gdbus-daemon', gdbus_daemon_sources],