Merge the wip/gapplication branch

This adds a GApplication object to GIO, which is the core of
an application support class, supporting
- uniqueness
- exporting actions (simple scripting)
- standard actions (quit, activate)

The implementation for Linux uses D-Bus, takes a name on the
session bus, and exports a org.gtk.Application interface.

Implementations for Win32 and OS X are still missing.

gio/gio.h

@@ -28,6 +28,7 @@
 #include <gio/giotypes.h>
 
 #include <gio/gappinfo.h>
+#include <gio/gapplication.h>
 #include <gio/gasyncinitable.h>
 #include <gio/gasyncresult.h>
 #include <gio/gbufferedinputstream.h>