Support withdrawing notifications

[cassidyjames]

Problem

Notifications stack up long after they're relevant, e.g. new mail notifications after all the mail is read, or AppCenter update notifications after you've updated

Proposal

We should support withdrawing notifications

Prior Art

I believe GNOME Shell supports the withdrawl API?

[Marukesu]

There's no withdraw method in GNotification, so I guess we should talk about what the developer story is for this feature

Isn't applications supossed to use GLib.Application.withdraw_notification for that?

[vjr]

One place from where this doesn't work: https://github.com/elementary/appcenter/blob/master/src/Core/Client.vala#L86