System tray back button is contextual, only visible when there's a place to go back to
Today on master, the system tray menu always shows a back button, regardless of the context.
But in many (most?) cases, the back button simply acts as a "close" button, since we are only "one layer" deep and there is nowhere to go back to.
I think a back button that sometimes closes the popup instead of going back is a confusing signal to a user.
This change introduces a property hasBack to the system track expanded representation, so we can signal when the back button is actually useful. It is set to true when we expand an applet from the items list and unset when the popup closes.
This is my second contribution, so I'm still learning the ins and outs :) Let me know if there's a better way to implement this.