Code review comment for lp:~gary-lasker/software-center/unity-launcher-integration-fixes

I took at look at Bilal's Unity side fix at https://code.launchpad.net/~bilalakhtar/unity/waiting-to-install-tooltip-925014/+merge/108858, and it looks to me to be approved now and ready for merging.

I noticed that his code is expecting an empty value for the transaction ID in the launcher dbus call in order to avoid displaying the "Waiting to install" tooltip, so I made that supporting change in this branch. Note that since we are firing the dbus call at the very end of the transaction now, this value will have no use at the launcher side anyway.

I've included a TODO in the code to document this for when we need to change this back.