Allow dragging app screenshots around

It is often the case that when clicking on an app screenshot, it is too big for the window that opens. So scrollbars are added. But scrollbars are an awkward interface to view more of a picture.

It would be more convenient to see more of the image with a mouse if you could click and drag the screenshot around inside its scrollbars. It would also be way more touch-friendly.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: software-center 5.2.7
ProcVersionSignature: Ubuntu 3.2.0-35.55-generic 3.2.34
Uname: Linux 3.2.0-35-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.1
Architecture: amd64
Date: Mon Jan 21 22:32:12 2013
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20120703-2
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20120703-15:08
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: software-center
UpgradeStatus: No upgrade log present (probably fresh install)