Add staging events from the qt-workflow-plugin to the gerrit event stream

In order for the upcoming cherry-pick bot to reliably stage changes together, it needs to receive staging and unstaging events from gerrit, as does with "change-merged" events and the like. While the bot is aware of cherry-pick staging events that it performs, in the event that a cherry-pick's direct parent must be staged manually (such as in the case of a merge conflict), gerrit does not presently send notifications on the event stream of staging events. As such, the bot cannot always stage changes together appropriately.

Suggested event names:

"change-staged", and "change-unstaged"