introduce GRemoteActionGroup

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

gio/Makefile.am

@@ -131,6 +131,7 @@ application_headers = \
 	gactiongroup.h			\
 	gactionmap.h			\
 	gsimpleactiongroup.h		\
+	gremoteactiongroup.h		\
 	gactiongroupexporter.h		\
 	gdbusactiongroup.h		\
 	gaction.h			\
@@ -152,6 +153,7 @@ application_sources = \
 	gactiongroup.c				\
 	gactionmap.c				\
 	gsimpleactiongroup.c			\
+	gremoteactiongroup.c			\
 	gactiongroupexporter.c			\
 	gdbusactiongroup.c			\
 	gaction.c				\