Fix Unity global menu and HUD

Changes by issue:
#869: `UBUNTU_MENUPROXY=1` and `GTK_MODULES=unity-gtk-module` is sufficient to enable global menus.
#802: The Unity HUD has been broken for a while. In trusty, fixing this requires inhibiting `gnome-settings-daemon` from running. Normally, Upstart would handle launching `unity-settings-daemon`. But since we're not using Upstart, `gnome-settings-daemon` is launched by its .desktop file, which wasn't updated to preclude it from starting in Unity. `window-stack-bridge` also needs to be started.

Other changes:
* `croutonversion -a` prints the chroot architecture.
* Consolidated Unity-related hacks into a couple of places so that scripts and desktop files don't need to be littered everywhere.
I believe desktop overrides are cumulative; i.e. this file only needs the OnlyShowIn line.