Pass an owned value and an egui::Ui to Plugin::run()

[abey79]

This PR changes the API of Plugin::run() in two ways:

• It pass an owned value of Plugin instead of a mutable reference. Based on my experience for similar APIs (e.g. Rerun's list_item), it can be very useful for implementer to have an owned value. For example it makes it possible for the plugin to carry a dyn FnOnce. This works because plugins are only ever called once. The trick to keep the trait object safe is to force the value to be wrapped in a Box. That's how things are already, so I figured this is acceptable.
• It pass an egui::Ui instead of an egui::Painter. This enables using egui to create more complex interactions within the view, for example hover tooltips etc. (see Provide Plugin::run() with an egui:Ui #197).

Fixes #197