Examples browser

Add a window that shows NodeBox example files, ordered by category.

Examples are stored in categories (Basic, Intermediate, Advanced) and sub-categories (e.g. under Basic: Shapes, Math, Color, e.g.). 

If their is a `png` file with the same basename as the `ndbx` file in the same directory, it is assumed to be the thumbnail.

The following are used from document properties:

- **title**: The user-readable example title.
- **description**: A longer description what this example is about (optional).
- **category**: The main category of the example (Basic, Intermediate, Advanced).
- **subCategory**: The underlying category of the example. Stored in the document properties.

Examples files should be read-only: you can change them, but saving them requires you save them under a different name.

Here is a screenshot of the current (rough) implementation:

![Screen Shot 2013-04-18 at 16 47 14](https://f.cloud.github.com/assets/8477/397102/e2a9a42a-a836-11e2-9e4f-8d81d09a7abd.png)