Add Panel tab with Indicator list

[cassidyjames]

Assuming we add an agent for indicators (see: https://github.com/elementary/wingpanel/issues/90), I presume we'll have a GSetting of allowed indicators. We would need to add somewhere to manage this after-the-fact, and a Panel tab in the Desktop settings makes the most sense to me.

Note: I don't think I want this in the Notifications settings because that implies it's a setting for apps to use for notifications. Instead, those apps should just use notifications. This is more of a desktop integration/system-type app that is extending the shell, so I feel like it belongs with the other shell settings.

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[cassidyjames]

I think the initial idea was to treat this like notifications as well, and require apps to put something in their .desktop file so we would know to show them here.