[Feature] make notifications follow focused monitor

[zjeffer]

Is your feature request related to a problem? Please describe.
When I'm focused on monitor A while a notification is still open on monitor B, the next notifications will also open on monitor B. I have to wait until the notification on monitor B disappears, before notifications will appear on monitor A.

Describe the solution you'd like
What I'd like is similar to dunst's 'follow' option. With this, you can set the notification to follow either the focus of the mouse, keyboard, or 'none'.

From dunst's documentation:

follow (values: [none/mouse/keyboard] default: none)

Defines where the notifications should be placed in a multi-monitor setup. All values except none override the monitor setting.

On Wayland there is no difference between mouse and keyboard focus. When either of them is used, the compositor will choose an output. This will generally be the output last interacted with.
none

• The notifications will be placed on the monitor specified by the monitor setting.
  mouse
• The notifications will be placed on the monitor that the mouse is currently in.
  keyboard
• The notifications will be placed on the monitor that contains the window with keyboard focus.

In my dunst config, I have this set to 'mouse'. This means that when a notification is present, if I then move my mouse to a different monitor, the notification will also be moved to that monitor.