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()`.

crates/egui_demo_app/src/backend_panel.rs

@@ -140,7 +140,7 @@ impl BackendPanel {
         {
             ui.separator();
             if ui.button("Quit").clicked() {
-                frame.quit();
+                frame.close();
             }
         }
     }