Merge lp://qastaging/~gary-lasker/software-center/launcher-integration-for-p into lp://qastaging/software-center

This branch implements the Software Center side of mpt's updated design for launcher integration functionality for Precise (https://wiki.ubuntu.com/SoftwareCenter#Learning_how_to_launch_an_application). It removes the explicit slide-in prompt in the details view for adding an item to the launcher, and instead always adds the item (or not) depending on the state of the new menu item "View->Add Applications in Launcher". A nice side-effect of the new design is that it now supports adding items to the launcher when they are installed from the details view.

There is also some refactoring to clean things up a bit and to set the stage for an easy update when the full Unity side implementation is ready. There's a new UnityLauncher class and you'll notice that I migrated the UI-side launcher integration code out of SoftwarePane and down into AvailablePane (there's no longer a reason for it to be in the superclass since we no longer use the ChannelPane).

The current implementation in the branch is designed to work using the Unity launcher dbus capabilities as they exist now. As such, it adds the application to the launcher once it has been installed, and does not yet display the install progress in the launcher. That functionality will come after the Unity side implementation is completed.

Finally, the supporting unit tests are updated and working.