eframe: rename quit/exit to "close" (#1943)

Since https://github.com/emilk/egui/pull/1919 we can continue
the application after closing the native window. It therefore makes
more sense to call `frame.close()` to close the native window,
instead of `frame.quit()`.

examples/custom_window_frame/src/main.rs

@@ -89,7 +89,7 @@ fn custon_window_frame(
                 Button::new(RichText::new("❌").size(height - 4.0)).frame(false),
             );
             if close_response.clicked() {
-                frame.quit();
+                frame.close();
             }
 
             // Interact with the title bar (drag to move window):