The plugin API of jquery.digilib
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* The plugin Javascript file has to be loaded after jquery.digilib.js.

* The plugin should be written as a closure on jQuery (like a jquery plugin) exporting a plugin object as its interface:

  var plugin = {
            name : 'mydigilibplugin',
            install : install,
            init : init,
            buttons : {},
            actions : {},
            fn : {},
            plugins : {},
            defaults : {}
  };

  The "name" string, "install" function and "init" function are provided by the plugin and called by digilib. 
  "buttons", "actions", "fn" and "plugins" are shared objects provided by digilib.
  "buttons" is the array of button objects. The plugin can add its own buttons to this array.
  "actions" is the array of action objects. The plugin can add its own actions to this array.
  "fn" is an object with functions from digilib.
  "plugins" is an object with all digilib plugin objects.
  "defaults" is an object with all digilib defaults.

* the plugin object is passed to digilib in the "plugin" action on loading the plugin javascript file:

  $.fn.digilib('plugin', plugin);

  digilib then calls the "install" function on the populated plugin object (i.e. this = plugin) with the plugin object as parameter.
  
* digilib calls the "init" function from the plugin object with the data object instance as a parameter in digilibs init phase.

* further actions should be implemented as custom events on the data object.

* events triggered by digilib on the data object:
  "setup": in the init phase, after scalerDiv and buttons have been set up.
  "update", updateDisplay(): after small updates in the display, e.g. when the scaler-img finished loading.
  "redisplay", redisplay(): after changes in the display, e.g. after changing zoom factor.
  "dragZoom(newZoomArea)": while dragging the zoom area (with parameter newZoomArea).