Default sidebar tab to notes when only page notes

Change the default tab to focus on page notes when there are
no annotations and only page notes in a group. This way it
directs the user where they need to go. Less clicking == yay!
If there are no annotations at all, the annotations tab should
be selected.

src/sidebar/reducers/selection.js

@@ -14,6 +14,7 @@ var immutable = require('seamless-immutable');
 
 var toSet = require('../util/array-util').toSet;
 var uiConstants = require('../ui-constants');
+var tabs = require('../tabs');
 
 var util = require('./util');
 
@@ -132,6 +133,17 @@ var update = {
     };
   },
 
+  ADD_ANNOTATIONS(state, action) {
+    var counts = tabs.counts(action.annotations);
+    // If there are no annotations at all, ADD_ANNOTATIONS will not be called.
+    var haveOnlyPageNotes = counts.notes === action.annotations.length;
+    // If this is the init phase and there are only page notes, select the page notes tab.
+    if (state.annotations.length === 0 && haveOnlyPageNotes){
+      return {selectedTab: uiConstants.TAB_NOTES};
+    }
+    return {};
+  },
+
   SET_FILTER_QUERY: function (state, action) {
     return {
       filterQuery: action.query,