Add BrowserWindow.isDevToolsOpened()

https://github.com/atom/electron/blob/master/docs/api/browser-window.md#winisdevtoolsopened

github-electron/github-electron.d.ts

@@ -356,6 +356,10 @@ declare module GitHubElectron {
 		 * Closes the developer tools.
 		 */
 		closeDevTools(): void;
+		/**
+		 * Returns whether the developer tools are opened.
+		 */
+		isDevToolsOpened(): boolean;
 		/**
 		 * Toggle the developer tools.
 		 */