Add an history of last QR Code generated/read in QR Code activity

[llaske]

Add an history of QR Code generated (or read). So the user could retrieve its history when he reopen the activity. The history could be represented by a sort of ComboBox in the input field.
The history should be stored in the datastore. See https://github.com/llaske/sugarizer/blob/dev/docs/tutorial_step4.md for more.
The source of the QR Code activity is located here: https://github.com/llaske/sugarizer/tree/dev/activities/QRCode.activity

[llaske]

Fixed in #250