GNOME: Add application wrapper and notifications UI

[michaelrsweet]

Similar to issue #27, add a tray icon and/or notifications for PAPPL-based printer applications on GNOME platforms. There should be a callback for bringing up a native printer application UI, otherwise choosing a printer or job should open the user's default web browser to the specified page.

[michaelrsweet]

Apparently current GNOME (vs. Ubuntu GNOME) has been trying to do away with application status icons, and the Ubuntu way relies on an undocumented and (apparently) unmaintained libappindicator library. So no status icon.

We can post notifications using the somewhat better documented GNOME notifications API, but that requires the use of GtkApplication or GApplication along with a DBUS listener to respond to any actions - not something I'm prepared to make as a requirement.

Deferring this issue but potentially track a future GUI wrapper around the WebKitGTK view so that a printer application just shows up as an icon in the "shell" (dock) and clicking on the icon or responding to a notification brings up the web interface.

[michaelrsweet]

Closing this as wontfix - 2.0 has removed the macOS menu bar extra stuff, and in general we'd want a separate application that then communicates with the printer application.