Clean up the docs for GApplication and related classes.
I'm no longer writing documentation for the structure type of classes
and interfaces. See https://bugzilla.gnome.org/show_bug.cgi?id=665926
for discussin on the correct way forward on this point.
Also: stop putting gtk-doc comments in installed headers.
Add support for passing the full contents of the environment to the
primary instance (by storing it in the platform_data) when
G_APPLICATION_SEND_ENVIRONMENT is in the flags.
