add scroll bars for instance/window switcher of application icon

[Karl-Philipp Richter]

When multiple instance of the same program and/or multiple windows of one program are opened, the icon in the starter offers a list of the instances and windows which allows to switch between them when the user right clicks on the icon. When more instances are opened than the maximum of entries in this list, the list just expands off the screen instead of offering scrollbars. This is a common issue when using `auto-apt` without privileges which opens a separate window for each invocation of `apt-get install` (running `auto-apt run ./configure` for gcc 4.6.4 easily makes `auto-apt` open > 50 terminals).

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: unity 7.3.1+14.10.20141016-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-25.33-generic 3.16.7
Uname: Linux 3.16.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.14.7-0ubuntu8
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CurrentDesktop: Unity
Date: Tue Dec 2 01:09:56 2014
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)