add universal egui context panel

*more details soon tm*

An issue at roseate is waiting for this to be completed: https://github.com/cloudy-org/roseate/issues/38

Context menu in GNOME's image viewer (loupe) as a reference:

![Screenshot_20241205_191657](https://github.com/user-attachments/assets/aea7d1c7-15d9-4f1d-b40c-01d24e43fa45)
