Inkscape extension: refactor import options dialog.

- Unify the main 'Default' and 'Custom' tabs into a single one.
- Use inline notebooks for color and page size options:  
  ```Default | Presets | Custom```
- Add presets for colors (background, foreground).  
  TODO: review presets, replace with more useful ones  

![before-after-1](https://cloud.githubusercontent.com/assets/4516067/5330077/fb554b14-7dd7-11e4-8342-a7e337118111.png)
(Screenshots done with Inkscape 0.91pre3 on OS X 10.7.5)