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

This feature is intended for clients that want to signal a desktop shell
their busy state, for instance because a long-running operation is
pending.
The API works in a similar way to g_application_hold and
g_application_release: applications can call g_application_mark_busy()
to increase a counter that will keep the application marked as busy
until the counter reaches zero again.

The busy state is exported read-only on the org.gtk.Application interface
for clients to use.

https://bugzilla.gnome.org/show_bug.cgi?id=672018

Now that we use an explicit list of symbols to export, the
G_GNUC_INTERNAL is redundant.

https://bugzilla.gnome.org/show_bug.cgi?id=688681

Provide public access to the GDBusConnect and object path that
GApplication is using.  Prevents others from having to guess these
things for themselves based on the application ID.

https://bugzilla.gnome.org/show_bug.cgi?id=671249

Use the fact that the action group exporter now speaks to
GRemoteActionGroup to get the platform data into GApplicion without
hacks.

https://bugzilla.gnome.org/show_bug.cgi?id=665737

This interfaceifies the extra functions that were on GDBusActionGroup
for dealing with platform data.

The two main benefits of doing this:

  - no longer have to do a silly song and dance in GApplication to avoid
    calling GDBusActionGroup API from non-dbus-aware code

  - the interface can be reused by the action group exporter to avoid
    ugly and unbindable hook callbacks

https://bugzilla.gnome.org/show_bug.cgi?id=665737

Use the _full variants of the GDBusActionGroup API to send platform data
for action invocations once again.

instead of the internal version of the same

Don't use GVariant* as the representation for the argument array.