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
Merge pull request #38 from su-v/update-inkscape-extension

Inkscape extension: refactor import options dialog.