Handle Notification Hints in the Server

[Marukesu]

this is a push to cleanup the Notification class to make it also usable with other server interfaces, hints are a exclusive concept of the Freedesktop's Notifications interface, and as such, it make more sense to deal with them in the server.

this introduce two behaviour changes:

• if app_icon is a file path or uri, it will be masked when used as primary image of the bubble, but not when used as a badge, this happens because there's no way to differ a GLib.FileIcon icon and a GLib.FileIcon image, given that the focus is the Gtk/Portal interface, i choose to not complicate the logic in the bubble to deal with this conner case.
• if none of app_name, summary or the desktop-entry hint is specified, we throw a error rather than showing a notification without title.