Introduce "More" button for StandardPanel

#### Purpose

Add "More" button to StandardPanel as specified in [UI Proposal](https://docs.google.com/document/d/1sPlXdh2vsxU7gVbW-iKm1s1JtGYNh9hnvbD8L7wiAww). Design with button is presented below.
![sp more btn](https://cloud.githubusercontent.com/assets/8158551/4247077/ef0e88c8-3a54-11e4-9531-45f24ee3bd22.PNG)

#### Modifications

Add "More" button to `LibraryView.xaml`. Introduced `IsMoreButtonVisible` for controlling `Visibility` property. Also added two methods `ShowMoreButton()` and `HideMoreButton()`.

#### Reviewers

@Benglin, please review.
In DataContextChanged we specify SecondaryMembers.ItemsSource. Also we change the whole `SecondaryMembers.ItemsSource` in `OnHeaderMouseDown`. I think it will be hard to work with `Observable<Collection>` for many add-remove actions.

So I didn't change this code.