eframe: Request window focus

[philpax]

Is your feature request related to a problem? Please describe.
I'm building an overlay that pops up (using frame.set_visible) on a hotkey with eframe and egui, and I'd like it to consistently focus the input widget. However, Response::request_focus does not appear to focus the widget the first time around.

I believe this is because the overlay window itself is not focused.

Describe the solution you'd like
eframe::Frame::request_focus, which requests the frame to be focused on platforms where it's supported.

I believe this would be relatively simple for the winit backend of epi: it should just be a call to winit::Window::focus_window.

Describe alternatives you've considered
Response::request_focus could implicitly focus the window, but that may be unexpected behaviour (especially as it would likely break existing workflows)

Additional context
I'm happy to implement this myself and submit a PR, but I wanted to check that this would fit within the purview of eframe first.

[TicClick]

this has been added in #2900, but not yet released under any tag