Added a new example, session-state (#300)

* Added a new example, session-state

* Review comments from andym

* Only remove the listener if this is our tab

session-state/README.md

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+# session-state
+
+## What it does
+
+This extension adds a context menu item. When the user clicks the context menu item, the extension adds a border to the page.
+
+But the extension also uses `sessions.setTabValue` to store the fact that it has added a border to this page. If the user closes the page, then restores it, the extension will retrieve this fact using `sessions.getTabValue`, and use that to reapply the border.
+
+Note: to restore a tab, press Control+Shift+T (or Command+Shift+T on a Mac). In Firefox you can also restore the tab from your "History->Recently Closed Tabs" menu.
+
+## What it shows
+
+This example demonstrates how you can use the [sessions](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/sessions) API to store and retrieve arbitrary state that you want to associate with a tab or window. Then if the tab/window is closed and subsequently restored, you can retrieve the state.

session-state/background.js

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+/**
+ * Add a context menu item to borderify the current tab.
+ */
+browser.menus.create({
+  id: "borderify-current-tab",
+  title: "Borderify current tab",
+  contexts: ["all"]
+});
+
+/**
+ * The borderify CSS.
+ */
+const borderCSS = 'body { border: 5px solid red };';
+
+/*
+ * Borderifies the current tab, and, using setTabValue, stores the fact
+ * that this tab was borderified.
+ */
+async function borderify() {
+  let tabArray = await browser.tabs.query({currentWindow: true, active: true});
+  let tabId = tabArray[0].id;
+  await browser.tabs.insertCSS(tabId, {code: borderCSS});
+  await browser.sessions.setTabValue(tabId, "border-css", borderCSS);
+}
+
+browser.menus.onClicked.addListener(borderify);
+
+/*
+ * When new tabs are created, we want to check, using getTabValue, whether
+ * the tab has the borderified state. That would mean it was borderified, then
+ * closed, then reopened, so we want to reapply the borderify CSS.
+ *
+ * But we can't insertCSS in onCreated, it's too early. So instead we make 
+ * an onUpdated listener, just for this tab. In onUpdated we check the tab ID,
+ * and if this is our tab, insert the CSS and remove the listener.
+ */
+browser.tabs.onCreated.addListener((tab) => {
+  
+  async function borderifyRestored(targetTabId, thisTabId) {
+    if (targetTabId === thisTabId) {
+      let stored = await browser.sessions.getTabValue(targetTabId, "border-css");
+      if (stored) {
+        let result = await browser.tabs.insertCSS(targetTabId, {code: stored});
+      }
+      browser.tabs.onUpdated.removeListener(thisBorderifyRestored);
+    }
+  }
+  
+  let thisBorderifyRestored = borderifyRestored.bind(null, tab.id);
+  browser.tabs.onUpdated.addListener(thisBorderifyRestored);
+});

session-state/icons/LICENSE

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+The icon “border-48.png” is taken from the Google Material Design iconset, and is used under the terms of the Creative Commons Attribution-ShareAlike license: http://creativecommons.org/licenses/by-sa/3.0/.

session-state/manifest.json

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+{
+  "applications": {
+    "gecko": {
+      "id": "session-state@example.com",
+      "strict_min_version": "57.0a1"
+    }
+  },
+  "manifest_version": 2,
+  "name": "Session state",
+  "version": "1.0",
+  "description": "Demonstrates using the sessions API to store and restore extension-specific tab state.",
+  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/session-state",
+  "icons": {
+    "48": "icons/border-48.png"
+  },
+
+  "background": {
+    "scripts": ["background.js"]
+  },
+  
+  "permissions": ["<all_urls>", "menus", "sessions"]
+
+}