Slint should have a show_fullscreen() function

[hunger]

As discussed in #2775 about documenting fullscreen mode:

We should have a show_fullscreen() API to slint::Window to control fullscreen mode programmatically.

Edit(@tronical): As outlined in #3331 , the acceptance criteria for this ticket should include show_maximized at the same time.

[JanDiederich]

It seems that both, most important window backends, Qt and Winit, have a native built in live go-to-fullscreen capability.

• Qt: https://doc.qt.io/qt-6/qwindow.html#showFullScreen.
• Winit, after a quick google: https://github.com/rust-windowing/winit/blob/master/examples/fullscreen.rs: window.set_fullscreen(fullscreen);

[ogoffart]

Implemented in https://github.com/slint-ui/slint/pull/4286