Bug#1025762: gnome-session: Install and enable appindicator and desktop icons NG extensions

[Jeremy Bicha]

Source: gnome-session
Version: 43.0-1

I propose that Debian enable 2 GNOME Shell extensions by default:
- gnome-shell-extension-appindicator
- gnome-shell-extension-desktop-icons-ng

Justification
----------------
Both those extensions fill gaps in our default desktop. For people
that use desktop icons or app status icons, these extensions are very
useful. For people that don't, the extensions can be basically
ignored.

Ubuntu includes these 2 extensions by default plus a variation of Dash to Dock.

Google has an internal distro for their developers that enables Dash
to Panel and appindicators. As of a few months ago, they were
considering adding Desktop Icons NG also.

I am expecting GNOME Classic, as early as GNOME 44, to re-enable
desktop icons by default and the Desktop Icons NG extension to be
moved upstream to the gnome-shell-extensions project.

There has been discussion about GNOME eventually enabling some sort of
app status icon feature by default, but work on the spec and
implementation appears stalled.

I think a discussion of adding Dash to Dock or Dash to Panel by
default should be discussed in a separate bug.

Implementation
---------------------
I suggest that we adapt similar work done by Ubuntu to add a GNOME
Shell mode and corresponding gnome-session files.

A GNOME Shell mode allows enabling specific extensions by default and
also allows enabling a few other customizations. GNOME itself uses a
GNOME Shell mode for GNOME Classic and the GNOME Initial Setup new
user mode. Ubuntu and Pop!_OS use GNOME Shell modes too.

The session identifies itself with the environment variable
XDG_CURRENT_DESKTOP=debian:GNOME

gsettings overrides can be set for just this debian session or for
GNOME sessions in general. For an example, see
https://git.launchpad.net/~ubuntu-desktop/ubuntu/+source/ubuntu-settings/tree/debian/ubuntu-settings.gsettings-override#n67

If anyone prefers the upstream experience, they can install the binary
package 'gnome-session' which I'm proposing we no longer install by
default.

This will need changes to our packaging for gnome-shell and
gnome-session. Later, we'll also need to patch gdm3 to prefer the new
Debian session instead of GNOME.

Thank you,
Jeremy Bicha