Improve performance of network graph

I think that loading network graph is as fast as that of 5.0.

### Overview

1. Rendering graph partially.
2. Reduce database access to get notes.
3. Enable vertical scrolling.Thanks @giggsey.
4. Some minor bugs related to network graph is fixed.

Dragging network graph is disabled, because it conflicts with scroll.

### Screenshot

![vertical-scrolling](https://f.cloud.github.com/assets/72138/428074/aa99b642-adf9-11e2-97c3-87d51dd69c4a.png)

fixes #3684
refs #3711