Merge the wip/gapplication branch

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.