Add close button to "this computer needs to restart" dialog

[Reuben Firmin]

See screenshot. What is this, Windows? Give users a choice about when they want to restart their computer (and apply kernel updates).

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: update-notifier 0.147.1
ProcVersionSignature: Ubuntu 3.11.0-17.31-generic 3.11.10.3
Uname: Linux 3.11.0-17-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.12.5-0ubuntu2.2
Architecture: amd64
Date: Sat Mar 8 08:30:54 2014
InstallationDate: Installed on 2014-02-24 (12 days ago)
InstallationMedia: Ubuntu-GNOME 13.10 "Saucy Salamander" - Release amd64 (20131017)
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-notifier
UpgradeStatus: No upgrade log present (probably fresh install)