checking/unchecking a bunch of updates is tremendously slow and blocks the UI

[Teo]

When you click on an item in the list of updates to check or uncheck it, it takes a riduculously long time to update the checkbox.
If you check or uncheck a whole group (e.g. Security Updates or Other Updates), it takes so long that the UI stops responding and greys out for a while.

There are two issue:
- this shouldn't take so long in the first place
- nothing that take significant time should ever, ever, block the UI, so it should be done in a separate thread so as to keep the UI always 100% responsive.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: update-manager 1:0.196.12
ProcVersionSignature: Ubuntu 3.13.0-34.60-generic 3.13.11.4
Uname: Linux 3.13.0-34-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.2
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Sep 21 13:18:20 2014
GsettingsChanges:
 b'com.ubuntu.update-manager' b'show-details' b'true'
 b'com.ubuntu.update-manager' b'window-height' b'748'
 b'com.ubuntu.update-manager' b'first-run' b'false'
 b'com.ubuntu.update-manager' b'window-width' b'677'
 b'com.ubuntu.update-manager' b'launch-time' b'1411288799'
InstallationDate: Installed on 2013-10-11 (344 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to trusty on 2014-05-24 (119 days ago)