Extract multi-transclude workaround into directive

The `multi-transclude` attribute directive encapsulates the angular 1.4
workaround for multiple transclusion targets pioneered in `kbn-top-nav`
for separation of concerns and re-use.

There are some differences to the implementation in `kbn-top-nav`:
* The directive logic is completely contained inside the link function
  and therefore won't interfere with other controllers.
* The slots are declared as the attribute value.
* The transcluded items are appended to the transclusion target's
  children instead of replacing the whole target. This preserves the
  attributes present on the target element.

src/ui/public/multi_transclude/multi_transclude.js

@@ -0,0 +1,51 @@
+import _ from 'lodash';
+import angular from 'angular';
+import uiModules from 'ui/modules';
+
+
+const module = uiModules.get('kibana');
+
+module.directive('multiTransclude', function MultiTransclude() {
+  return {
+    link: linkMultiTransclude,
+    restrict: 'A',
+    scope: {
+      'multiTransclude': '=',
+    },
+  };
+});
+
+function linkMultiTransclude(scope, element, attrs, ctrl, transclude) {
+  const transclusionSlotNames = scope.multiTransclude;
+  const transcludes = {};
+
+  transclude(clone => {
+    // We expect the transcluded elements to be wrapped in a single div.
+    const transcludedContentContainer = _.find(clone, item => {
+      if (item.attributes) {
+        return _.find(item.attributes, attr => {
+          return attr.name.indexOf('data-transclude-slots') !== -1;
+        });
+      }
+    });
+
+    if (!transcludedContentContainer) {
+      return;
+    };
+
+    const transcludedContent = transcludedContentContainer.children;
+    _.forEach(transcludedContent, transcludedItem => {
+      const transclusionSlot = transcludedItem.getAttribute('data-transclude-slot');
+      transcludes[transclusionSlot] = transcludedItem;
+    });
+  });
+
+  // Transclude elements into specified "slots" in the top nav.
+  transclusionSlotNames.forEach(name => {
+    const transcludedItem = transcludes[name];
+    if (transcludedItem) {
+      const transclusionSlot = document.querySelector(`[data-transclude-slot="${name}"]`);
+      angular.element(transclusionSlot).append(transcludedItem);
+    }
+  });
+}