lokdocview: use lok::Office::registerCallback()
lokdocview: use lok::Office::registerCallback()

This way we get some feedback on the state of the loading at least on
the console.

Change-Id: I95bf6cebcdd8b879c817b4e027d4f5b4acb9cd4c

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core