Bug 858399 - solution adds context menu to panels

Context menu only shows when contextMenu: true in panel constructor (defaults to false)

Also added get contextMenu property from panel

Also added ability to temporarily change contextMenu property setting in
the panel show(options) function

EXAMPLE USAGE:
```js
var { ToggleButton } = require('sdk/ui/button/toggle');
var panels = require("sdk/panel");

var button = ToggleButton({
  id: "my-button",
  label: "my button",
  icon: {
    "16": "./icon-16.png",
    "32": "./icon-32.png",
    "64": "./icon-64.png"
  },
  onChange: handleChange
});

var panel = panels.Panel({
  contentURL: "https://news.ycombinator.com",
  width: 500,
  height: 600,
  onHide: handleHide,
  contextMenu: true   //initial contextMenu setting, defaults to false
});

function handleChange(state) {
  if (state.checked) {
    panel.show({
      position: button,
      contextMenu: false  //override contextMenu constructor setting temporarily
    });
  }
}

function handleHide() {
  console.log("panel.contextMenu:"+panel.contextMenu);  //read the current contextMenu setting
  button.state('window', {checked: false});
}
```