Support for pausing notifications while in fullscreen

[LRitzdorf]

While we can already use the layer config option to control showing notifications on top of a fullscreen window, it'd be nice to have the ability to pause notifications instead, and show them upon exiting fullscreen mode.

This is heavily inspired by dunst's fullscreen option:

"fullscreen"
One of show, delay, or pushback.

This attribute specifies how notifications are handled if a fullscreen window is focused. By default it's set to show so notifications are being shown.

Other possible values are delay: Already shown notifications are continued to be displayed until they are dismissed or time out but new notifications will be held back and displayed when the focus to the fullscreen window is lost.

Or pushback which is equivalent to delay with the difference that already existing notifications are paused and hidden until the focus to the fullscreen window is lost.

On wayland, if follow is set to mouse or keyboard, the output where the notification is located cannot be determined. So dunst will delay or pushback if any of the outputs is fullscreen. Since the fullscreen protocol is fairly new, you will need a recent version of a compositor that supports it. At the time of writing, you will need the git version of sway. See also layer to change if notifications appear above fullscreen windows in Wayland.

This will probably depend on one or another Wayland protocol (dunst's docs for this feature mention needing a recent compositor, but don't state the relevant protocol).