Context panel (a universal one derived from the cirrus tool kit)

[THEGOLDENPRO]

Most applications like image viewers typically have a panel that opens upon right-click events with an array of buttons to trigger various features in the application quicker.

It's so common that many new users of roseate will expect such a feature.

However for the context panel to be consistent with future cloudy-org applications we should create the egui widget in the cirrus tool kit. An issue is already open for this at the cirrus repo: cloudy-org/cirrus#3

Context menu in GNOME's image viewer (loupe) as a reference: