Add support for setting 'Exclusive' full screen when clicking on maximise

Hello,

I'm currently working on a game that runs much better in exclusive full screen mode. I think this is because the framebuffer doesn't need to be copied and re-scaled to match the DPI settings of the monitor.

Anyway, I see that fullscreen was added in this pull request. There are two ways to interact with it:

1. You can use WindowBuilder::with_fullscreen to set exclusive mode when the window is first created
2. You can call Window::set_fullscreen to toggle it later

What I really want is for the game to first launch normally in a window, but then if the user clicks on maximise it enters exclusive full screen mode. I can't see that there's a way to do this?

Currently I'm using winit version 0.21.0 on macOS Catalina 10.15.6. The native resolution of my display is 2560x1600 and with DPI scaling this is reported as 2880x1800. This is what I mean by the 'maximise' button:

I could try to detect when this has happened by checking window.inner_size() after a resize event and comparing that to the monitor's video modes but this seems like it might be brittle.

Would it be possible to add a setting that's something like...

WindowBuilder::set_maximise_behaviour(...)

...that allows me to configure how that button should be handled? Perhaps it could fall back to the default behaviour on platforms that don't support exclusive full screen mode?

Start by reviewing WindowBuilder::with_fullscreen and Window::set_fullscreen, including their platform-specific behavior and the existing maximise handling. Determine how the maximise button is represented across supported platforms and define the fallback behavior; done means the requested setting reliably selects exclusive fullscreen where supported without changing default behavior elsewhere.