Should be possible to have a MenuItem with subitems, mainly to group operations.

Use-case: 

- Group similar operations under a single menu entry as not to bloat the context
menu

Example:

- For anyone that used TortoiseSVN, when you right-click a file inside a
directory that was checked out from SVN, you get a menu like:

  - SVN Add
  - SVN Commit
  - SVN Operations
    \- Remove 
    \- Check for Modifications
    \- Preferences
    \- ...     

If all the entries were top-level, the menu could easily end up with 30 menu
entries, no matter how smart your extension is to hide non-meaningful operations.

There are other cases where this would be useful:

A Zip menu, like WinZip, but using file-roller:

- Zip (some better name?)
  \- Add to existing archive
  \- Zip & Email
  \- Unzip to current folder
  \- Unzip to <dirname>

A virus scanner menu:

- Virus Scanner
  \- Check for viruses
  \- Clean viruses
  \- Quarantine

A Send-To menu:

- Send-To
 \- Email Recipient
 \- My Images Folder
 \- <folderish-target bookmarks from nautilus>
 \- Gaim Recipient

'campd' mentioned on #nautilus that now that the ui manager is being used, doing
this should be fairly simple.