Shared ng-models state for nested models

[spalger]

As noticed in #3518 (comment), our number-list control uses multiple ng-models, causing it to feel like seperate inputs when it should feel like a single input.

Currently This is expressed in the markup by having an ng-model directive on the kbn-number-list directive. The template for that directive though creates several nested ng-models (one for each list item) and they all maintain individual state (valid, invalid, touched, etc.)

<!-- kbn-number-list is a control that should operate on "agg.params.values" -->
<kbn-number-list ng-model="agg.params.percents">
  <!-- kbn-number-list template -->

    <!-- for each item in the list -->
    <div ng-repeat="value in list track by $index">

      <!-- input a single number -->
      <input ng-model="list[$index]" kbn-number-list-input>

    </div>

  <!-- end kbn-number-list template -->
</kbn-number-list>

I think we need to make these changes:

1. sub-ng-models should not be a part of the form, but should control the state of the parent ng-model. This way, the form's "invalid controls" will simply contain a single model, the parent, and not a model for each invalid child model.
2. sub-ng-models should sync their "touched" state with the parent, which should then propagate the state back to all other children
   
3. sub-ng-models should sync their "validity" and "pristine" states with the parent, but the parent should not propagate these states.
   

[timroes]

I think we can close that, since we are a) in the process of building a new editor (#17076) and b) replacing everything by React (#16728). Please feel free to reopen if you think this is still an issue, that we need to touch beforehand.