fix buggy style for nautilus on ubuntu 16.04 flashback (light-theme)

[Wei Tsui]

1) Disable CDS style when XDG_CURRENT_DESKTOP="GNOME-Flashback:Unity"

2) Show the menubar like it is shown under Unity Session.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: nautilus 1:3.18.4.is.3.14.3-0ubuntu4
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME-Flashback:Unity
Date: Sat Apr 23 15:51:00 2016
GsettingsChanges:
 b'org.gnome.nautilus.window-state' b'geometry' b"'1105x816+176+136'"
 b'org.gnome.nautilus.desktop' b'trash-icon-visible' b'false'
 b'org.gnome.nautilus.list-view' b'default-column-order' b"['name', 'size', 'type', 'date_modified', 'date_accessed', 'owner', 'group', 'permissions', 'mime_type', 'where']"
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
SourcePackage: nautilus
UpgradeStatus: No upgrade log present (probably fresh install)
usr_lib_nautilus:
 file-roller 3.16.4-1ubuntu3
 gnome-terminal 3.18.3-1ubuntu1