Refactored Piwik support for better readability and extensibility. Added a couple more custom variables (Sort, Page, Limit, View) to search tracking.

- Resolves VUFIND-1029.

- We always show a drop-down, so there's no need for conditional link text.

- Thanks to Ere Maijala for catching these.

- Added QR code test
- Adjusted QR code loader to use more consistent public interface

- Includes tests.