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/tests/appinfo-test.desktop

@@ -0,0 +1,4 @@
+[Desktop Entry]
+Type=Application
+Name=appinfo-test
+Exec=./appinfo-test