Add "Save to Zotero" submenu to the Firefox content area context menu.  ...

...Use the same menupopup as used for the status bar icon for the submenu.  This addresses zotero/zotero#356 .

A couple comments:

* I used the same context menu generating function used by the status bar icon for the content area context menu.  I think this is fine, but in the future it could be confusing development-wise that the `onStatusPopupShowing` function is also used for the content area context menu.  I added a comment to `onStatusPopupShowing` to point this out.

* I set the hidden status of the submenu of the content area context menu based on the hidden status of `statusImage.hidden`.  I think this is okay because it should always be set before the context menu can be shown, but again the name could be confusing.

* I only added the context menu.  A "Save to Zotero" menuitem equivalent to clicking on the status bar icon could also be added directly to the Zotero submenu but maybe it's simpler just to have the "Save to Zotero" options in the separate submenu of the Zotero submenu.