GApplication: parse command line options

Add support for parsing command line options with GApplication.

You can add GOptionGroup and GOptionEntry using two new APIs:
g_application_add_option_group() and
g_application_add_main_option_entries().

Also add a "handle-local-options" signal that allows handling of
commandline arguments in the local process without having to override
local_command_line.

As a special feature, you can have a %NULL @arg_data in a GOptionEntry
which will cause the argument to be stored in a GVariantDict.  This
dictionary is available for inspection and modification by the
"handle-local-options" signal and can be forwarded to the primary
instance in cases of command line invocation (where it can be fetched
using g_application_command_line_get_options()).

https://bugzilla.gnome.org/show_bug.cgi?id=721977

gio/gapplicationimpl.h

@@ -31,7 +31,7 @@ void                    g_application_impl_open                         (GApplic
                                                                          GVariant           *platform_data);
 
 int                     g_application_impl_command_line                 (GApplicationImpl   *impl,
-                                                                         gchar             **arguments,
+                                                                         const gchar *const *arguments,
                                                                          GVariant           *platform_data);
 
 void                    g_application_impl_flush                        (GApplicationImpl   *impl);