Add state for navigation

Currently selected navigation item is available under the navigation
variable so that the whole application has access to it.

app/components/__tests__/EntrySelector-test.jsx

@@ -12,6 +12,6 @@ describe('EntrySelector', () => {
         var entrySelector = TestUtils.renderIntoDocument(
             <EntrySelector id="entry-selector" className="left inline fill-height" />
         )
-        expect(entrySelector.state.entries.length).toEqual(0)
+        expect(entrySelector.state.notes.length).toEqual(0)
     })
 })