enabled and visible should be allowed to be functions

[mar10]

• New option 'visible', that could be a function returning bool
• New option 'enabled', that could be a function returning bool
• Deprecate 'disabled' ? At least check that it is not mixed with 'enabled'
• ? New option 'status', that could be a function returning 'enabled' | 'diabled' | 'hidden'

{title: "Copy", cmd: "copy", uiIcon: "ui-icon-copy", 
    visible: function(event, ui) { return ...; }
    enabled: function(event, ui) { return ...; }
},

[mar10]

There should be one-- and preferably only one --obvious way to do it.

Since it is already possible to disable/hide with existing methods, I close this for now.

[mar10]

The current implementation will allow to pass a callback function as disabled option.
It will be called as fn(event, ui) and should return true, false, or the string "hide".

Dynamic text may be defined by passing a similar callback for the title option.