fixed a bug that upstream scratch-gui has (attempt to save project with ctrl+s even if !canSave)

Author: Titanium2099

src/components/menu-bar/menu-bar.jsx

@@ -338,8 +338,10 @@ class MenuBar extends React.Component {
     handleKeyPress(event) {
         const modifier = bowser.mac ? event.metaKey : event.ctrlKey;
         if (modifier && event.key.toLowerCase() === 's') {
-            this.props.onClickSave();
-            event.preventDefault();
+            if(this.props.canSave){
+                this.props.onClickSave();
+                event.preventDefault();
+            }
         }
     }
     getSaveToComputerHandler (downloadProjectCallback) {