(eframe) add Frame.set_visible (#1808)

eframe/src/web/backend.rs

@@ -300,6 +300,7 @@ impl AppRunner {
                 decorated: _,    // Can't toggle decorations
                 drag_window: _,  // Can't be dragged
                 window_pos: _,   // Can't set position of a web page
+                visible: _,      // Can't hide a web page
             } = app_output;
         }