message: Adapt contextMenu in background.js
  Contextmenu will appear only when Sideex executing.
diff: @@ -25,6 +25,33 @@ function openPage() {
  console.log(`Error: ${error}`);
  });
  + browser.contextMenus.create({
  + id: "verifyText",
  + title: "verifyText"
  + });
  + browser.contextMenus.create({
  + id: "verifyTitle",
  + title: "verifyTitle"
  + });
  + browser.contextMenus.create({
  + id: "assertText",
  + title: "assertText"
  + });
  + browser.contextMenus.create({
  + id: "assertTitle",
  + title: "assertTitle"
  + });
  + browser.contextMenus.create({
  + id: "storeText",
  + title: "storeText"
  + //contexts: ["all"]
  + });
  + browser.contextMenus.create({
  + id: "storeTitle",
  + title: "storeTitle"
  + //contexts: ["all"]
  + });
  +
  }
  browser.browserAction.onClicked.addListener(openPage);
  @@ -46,6 +73,8 @@ browser.windows.onRemoved.addListener(function(windowId) {
  querying.then(disconnectAllTabs, queryError);
  panelId = undefined;
  }
  +
  + browser.contextMenus.removeAll();
  });
  function passingWindowId(window_id, sideex_id) {
  @@ -75,33 +104,6 @@ function passingWindowId(window_id, sideex_id) {
  });
  };
  -browser.contextMenus.create({
  - id: "verifyText",
  - title: "verifyText"
  -});
  -browser.contextMenus.create({
  - id: "verifyTitle",
  - title: "verifyTitle"
  -});
  -browser.contextMenus.create({
  - id: "assertText",
  - title: "assertText"
  -});
  -browser.contextMenus.create({
  - id: "assertTitle",
  - title: "assertTitle"
  -});
  -browser.contextMenus.create({
  - id: "storeText",
  - title: "storeText"
  - //contexts: ["all"]
  -});
  -browser.contextMenus.create({
  - id: "storeTitle",
  - title: "storeTitle"
  - //contexts: ["all"]
  -});
  -
  var port;
  browser.contextMenus.onClicked.addListener(function(info, tab) {
  port.postMessage({ cmd: info.menuItemId });
  