(eframe) Way to focus window

This should be fairly easy to do with [focus_window](https://docs.rs/winit/latest/winit/window/struct.Window.html#method.focus_window).

Start by tracing the native window integration in eframe and reading the linked winit Window::focus_window documentation. Determine where a public way to focus the application window belongs, then verify that the behavior works for the supported native targets.

Written by the indexing model from the issue text.