enh(ui): add single signal slot for notifications popups

Description

Simplify notification API by using single signal & slot

• One signal "notified" in IService containing the type & the message
• One slot "pop" in SNotifier reading type & message and display corresponding notification popup.

⚠ breaking changes

Closes #925 (closed)

How to test it?

Modification has been made in ExNotifications to test it