Overhaul of powerd implementation required for consolidating scattered power policy functionality from other projects

We have some functionality related to power policy living in various projects, e.g. unity-system-compositor among others. In discussions with tvoss, we have concluded that this functionality should live in powerd, not scattered amongst various projects.

As things stand, it's difficult for, say, USC to make power policy decisions on its own. We have some bugs related to power policy against USC that we (the Mir team as the owners of USC) are not sure how to address.

However, according to tvoss, before scattered functionality can be consolidated, powerd itself has to be overhauled to allow easier feature additions/modifications. tvoss has already started some branches for this, but they are incomplete. This is the motivation for this bug report.

Since some of the power related bugs scattered in various projects that are presently open have 'High' priority, it'd be appropriate to make this bug at least the same priority as this is a prerequisite.