Add "call" category to notification spec

[Jared Toomey]

Hello all,

I would like to start a discussion for adding a "call" category and
relevant subcategories to the freedesktop notification-spec:
(specifications.freedesktop.org/notification-spec/latest/ar01s06.html).

There's no appropriate standard category for an incoming call. 
The mobile Wayland shell Phosh calls `feedbackd` to trigger haptic
feedback and play notification sounds depending on the current profile.
It checks the notification category to trigger the correct feedback.

The XMPP client Dino uses generic category "im" for calls, which is not
in Phosh's hardcoded list of categories that trigger feedback.
As a result, there is no sound or vibration for incoming calls.

Tweaking Phosh to trigger `phone-incoming-call` feedback for the 
"im" category allows Dino to trigger the ringtone and vibration.
This is just a hack; any other apps using the "im" category more
appropriately would trigger a full ringtone instead of a short alert.

I'd imagine many other apps could take advantage of a calls category.
It is an easy way to integrate feedbackd on mobile Linux.

GNOME has made "x-gnome.call.unanswered" for missed calls, though I'm
not sure what the logic there is; seems "im" would suit that type of
notification. It may be worth standardizing as well I suppose.

Kind regards,

Jared Toomey