[#211] Read 'allowMultiple' attr from question
[#211] Add UI widgets

* Show stored values
* Keep the single barcode consistent
[#211] Store multiple values

* Update UI.
* Read/Write pipe-concatenated values.
[#211] Externalize resources
[#211] Hide blank input on read-only mode
[#211] Update icons
Merge pull request #220 from akvo/issue/211-multiple-scan

Issue/211 multiple scan