Fix Cinnamon target after Cinnamon 2.0 release

Cinnamon 2.0 made some big changes that break Cinnamon on crouton. `gnome-session` no longer works, and `cinnamon-session` needs to be used instead. There's also a dependency in the `cinnamon.session` file on `nm-applet`, so this patch adds a dummy `/usr/local/share/applications/nm-applet.desktop` that will appease `cinnamon-session`. See #427

This also removes Cinnamon on ARM, which has gotten feedback that it's unusable and slow because of its 2D software rendering fallback.