feat: Option to show notifications over fullscreen windows

[hojjatabdollahi]

If you're watching a video or palying a game in fullscreen you'd miss the "Battery Low" notification.

This PR adds an option to show None/All/UrgentOnly notifications over fullscreen windows.

Methodology:
In cosmic-comp Top layers are drawn behind the fullscreen layer and overlay is drawn over it.
By default all notifications are shown on a Top layer (current behavior) which means they're covered by a fullscreen surface. This draws the notifications on Top or Overlay surface depending on the config.

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