Add a menu entry based on the clipboard content if it is an existing folder or file

[JnLlnd]

See: http://code.jeanlalonde.ca/folderspopup/#comment-11804

[JnLlnd]

A few notes or questions:

• Adding a menu on the fly puts the added menu at the end of the menu (unless I rebuild the menu which would cause an unwanted delay before showing). If we want the clipboard menu at a specific location, it must be created at startup and updated/enabled/disabled at will at each call to the menu.
• Assuming the clipboard menu would be permanent, what if when the popup menu is called, the clipboard does not contain a file or folder? Should the previous menu be kept or should the entry be disabled?
• I would add an option for users who would not want this new menu entry.

[JnLlnd]

Done in v4.9.2