When using Nuxeo.DocumentContentBehavior I need to be able to set the value of `params` in a more flexible way

Currently Nuxeo.DocumentContentBehavior uses a computed binding to set the value of `params`. This poses a couple of problems:

Problem 1: You cannot set the value of a property that has a computed binding directly. I.e. can't do something like:

observers: ['_setParams(currentUser)'],
_setParams: function(currentUser) {
  if (currentUser && currentUser.id) {
    this.params = { "alert_user": currentUser.id }
  }
},

Problem 2: the computing function only observes the `document` property. This is too restrictive as the page provider may have other parameter values that don't depend on the current document. So if you have:

_computeParams: function(document) {
    return document ? { 'ecm_parentId': document.uid, 'someParam': myCustomParam } : {};
}

There's no guarantee `myCustomParam` has a value since the function only runs when the `document` property changes.