Reduce code duplication for context menu items

The current getBarcodeFromLink/getBarcodeFromSelection functions
actually open new tabs rather than returning information, and there is
duplication of code between the get*/show* functions.

1. Rename getBarcodeFromSelection to openBarcodeFromSelection
2. Rename getBarcodeFromLink to openBarcodeFromLink
3. Add new functions for getBarcodeURLFromSelection and
   getBarcodeURLFromLink
4. Use the new functions in the open*/show* functions instead of
   having duplicated code
Merge pull request #4 from Sophira/less-duplication

Reduce code duplication for context menu items