added a cancel option to popups (closes #28)

src/js/popup.js

@@ -77,15 +77,20 @@ class Popup {
 		buttons.forEach(btn => buttonSection.appendChild(btn));
 
 		parent.appendChild(outer);
+		Popup.displayed.push(this);
 
-		const code=await Promise.race(buttons.map(btn => new Promise(ok => {
+		const btnActions=buttons.map(btn => new Promise(ok => {
 			btn.addEventListener('click', e => {
 				e.preventDefault();
 				return ok(btn.dataset.code);
 			});
-		})));
+		}));
+		const dismissAction=new Promise(ok => this.dismiss=ok);
+
+		const code=await Promise.race(btnActions.concat([dismissAction]));
 
 		parent.removeChild(outer);
+		Popup.displayed.splice(Popup.displayed.indexOf(this), 1);
 		return code;
 	}
 }
@@ -93,4 +98,9 @@ class Popup {
 Popup.EM=Symbol('EM');
 Popup.STRONG=Symbol('STRONG');
 
+Popup.displayed=[];
+Popup.dismiss=arg => {
+	Popup.displayed.forEach(p => p.dismiss(arg));
+};
+
 return module.exports=Popup;