Add support for hints in notification portal

[grulja]

Add support for: https://developer.gnome.org/notification-spec/#hints

That way we should be able to pass custom properties through the notification portal. E.g. in KDE we use many custom properties that are just passed in "hints" as a simple variant map. It shouldn't be hard to support that, this might just be one more specified option in "notification" parameter in AddNotification method.

E.g. hints a{sv} - array of extra properties, the backend implementation is not required to support any of them

[matthiasclasen]

the fundamental problem with such optional hints is that apps can't rely on them, and don't know what the user experience will be.

[TingPing]

I don't think that is a real problem, the fdo interface allows the client to query supported features, this just needs the same.

[Mikenux]

There is a new proposal to discuss for notifications: #983

[RokeJulianLockhart]

#77 (comment)

@grulja, https://developer.gnome.org/notification-spec/#hints has been replaced by https://developer-old.gnome.org/notification-spec/#hints.