Improved data selector

- Visually improved
- Radio buttons for toggling sequence / other track selection
- Guided selection: first the sequence, then other data (see attached images
- Scrollbar appears in project list if it is too long

At first:
![jbrowse-selector-1](https://cloud.githubusercontent.com/assets/606492/3380481/3d3b8780-fc07-11e3-8ac9-6d84c35dda06.png)

When sequence track selected, the 'Alignment data' button is enabled:
![jbrowse-selector-2](https://cloud.githubusercontent.com/assets/606492/3380484/486565a4-fc07-11e3-8edd-fe99d5faabab.png)