Updated file importer interface and fixed Spreadsheet import

Spreadsheet import through the generic file importer always failed,
since the books cannot be imported to Development Project. Now the
generic file interface takes this into account and provides the
selection as a resource. Currently only ExcelFileImport uses

Amend 1: Changed the way the selected resource is found
Amend 2: Take into account Tuukka's review comments (1,2,3 and 5)
Amend 3 and 4: Changes by Jani and Miro to fix some issues with the
interface

gitlab #53
gitlab #56

Change-Id: Ibfb9d54d8c36a3dc2aa7da8f1043613159ae8383