Don't close window with Cmd-W on first tab if other tabs are open

Closes #2572

chrome/content/zotero/tabs.js

@@ -52,6 +52,10 @@ var Zotero_Tabs = new function () {
 		get: () => document.getElementById('tabs-deck')
 	});
 
+	Object.defineProperty(this, 'numTabs', {
+		get: () => this._tabs.length
+	});
+
 	this._tabBarRef = React.createRef();
 	this._tabs = [{
 		id: 'zotero-pane',

chrome/content/zotero/zoteroPane.js

@@ -795,6 +795,15 @@ var ZoteroPane = new function()
 	}
 	
 	
+	this.handleClose = function (event) {
+		// Don't close the window from the first tab if other tabs are open
+		if (Zotero_Tabs.numTabs > 1) {
+			return;
+		}
+		window.close();
+	};
+	
+	
 	/*
 	 * Highlights collections containing selected items on Ctrl (Win) or
 	 * Option/Alt (Mac/Linux) press

chrome/content/zotero/zoteroPane.xhtml

@@ -95,7 +95,7 @@
 	<commandset id="mainCommandSet">
 		<!--FILE-->
 		<command id="cmd_quitApplication" oncommand="goQuitApplication(event);"/>
-		<command id="cmd_close" oncommand="window.close();"/>
+		<command id="cmd_close" oncommand="ZoteroPane.handleClose(event)"/>
 		
 		<!--EDIT-->
 		<command id="cmd_find"