add an indirect mode to AlternativesView

Adds a way to specify a direct or indirect mode. Direct mode behaves as before.

In indirect mode, a signal is emitted instead of directly proceeding to the createJob when the button is clicked, such that applications have time to create the document on demand that is going to be shared . The problem in Skanpage is that the document may not exist yet when invoking the share view as it is only a collection of scanned images. The user can select between PDF and different image formats, and then the document is created on demand.