[Notifications] Add quick reply feature

This adds a quick reply feature with a text field inline in the notification popup.
An action named "inline-reply" will spawn the text field and a NotificationReplied signal is emitted then.
There's additional kde hints for changing the placeholder text (defaults to "Type a reply..."),
submit button text (defaults to "Send") and submit button icon name (defaults to "document-send",
that paper aeroplane icon).