* Add Dear ImGui example
- Closes#90
* Change argument order to match `render_with`
* Remove unnecessary borrow
* Refactor error messages
* Add a space
* Refactor Gui field privacy
* Add a menu bar and allow the about window to be closed
- The local bool is necessary because the menu bar closures are not allowed to borrow `self` for mutable access while `imgui::Ui<'ui>` is alive.
- The token-based menu bar lifetime is even more verbose than this.
