Allow modules to customize/override the visual representation

[skaldarnar]

Currently the only customization options for Notifications are to provide overrides or deltas for assets/ui/NotificationAreaOverlay.ui and/or the currently used skin engine:default. This allows to move the notification area to a different place on the screen, or change the background of the notification boxes.

However, it does not allow to change the size of the message boxes, the layout in which the elements are rendered, or whether notifications are stacked top-to-bottom, bottom-to-top, ...

Suggestion

• Define layout in .ui asset Instead of just using a custom notification rendered which defines the layout hard-coded we may introduce some kind of widget as .ui file which could be modified from other modules. The item renderer would then load this prefab (once when initialized), do a lookup of predefined fields/widgets (i.e., title, subtitle, and icon), and fill in the respective content from the notification to be rendered.

• Provide custom .skin file We should also provide a custom skin that is specific for the notification area to make adjustments of the skin possible without changing the renderer or .ui asset.

This is related to #2