Unify menus with tooltips/popups

[emilk]

egui menus (Ui::menu_button) and context menus (Response::context_menu) are implemented using the egui::menu module.

There are also tooltips (Response::on_hover_ui etc) and ComboBox popups, both implemented using egui::containers::popup.

It would be nice to unify these systems.

Things I would like to accomplish:

• Being able to open a popup on:
  • hover (tooltip)
  • click (menu)
  • right-click or long-press (context menu)
• Nest sub-menus everywhere (currently not possible in tooltips)
• Call ui.close_menu to close a tooltip and popup (currently only works for menus)

At the same time it would be great to take a naming pass on all these things, and look up what they are commonly called in other ui libraries.

[Umatriz]

I would like to try to implement this. But I also want to be able to change the way egui::containers::popups are closed. Because for now if I add any kind of checkboxed into it or anything else that requires some kind of interaction the popup will immediately close as soon as I change something (mark a checkbox or press a button). I think it's better to let user to take control of it. Simitarly to the ui.menu_button and it's ui.close_menu.

[Umatriz]

I think I will make a PR and write all my thoughts about this isssue.

[emilk]

Nice! I suspect there is a lot in menu.rs that can be simplified too