Rewrite Budgie PanelManager to support panels across multiple-monitors

[JoshStrobl]

While planned for Budgie 11, based on the Making Budgie 10 Fly Again discussion it is quite clear that we all would like a rewritten panel manager for support panels across multiple-monitors, so this is a task to provide an opportunity to get to the work on that.

This implementation would need to be aware of laptop docking, random issues where a monitor might come unplugged and thus the panel / display is lost, resolution changes, etc.

[serebit]

Worth noting is the fact that we cannot extend the X system tray to any monitors other than the primary. This is not a limitation of CarbonTray, but a limitation of X itself. Because of this, we cannot copy a panel configuration over to secondary monitors without modification.

[JoshStrobl]

@serebit I think this would further emphasize the need to support the StatusNotifierItem specification, which is over D-Bus and supports multiple StatusNotifierHost (things that draw the items).

[gravityfargo]

This would be a massive quality of life improvement for me!