Bug 858399 - solution adds context menu to panels

Context menu only shows when contextMenu: true in panel constructor
(defaults to false).

contextMenu is now added to the panelContract, not the displayContract

Also added get contextMenu property from panel

Removed contextMenu from show() and relaced with ability to set
contextMenu property (i.e. panel.contextMenu=true)
Nit: our [coding style guidelines](https://github.com/mozilla/addon-sdk/wiki/Coding-style-guide#conditionals) specify – unfortunately, 'cause I have your same style here – that `else` should be on a new line, if you're using braces. So, either:
```js
if (allow) {
  panel.setAttribute("context", "contentAreaContextMenu");
}
else {
  panel.removeAttribute("context");
}
```
or:
```js
if (allow)
  panel.setAttribute("context", "contentAreaContextMenu");
else
  panel.removeAttribute("context");
```